Process for Flexible and Shape-Conformal Rope-Shape Alkali Metal-Sulfur Batteries

ABSTRACT

Provided is a process for producing a rope-shape alkali metal-sulfur battery, comprising (a) providing a first electrode comprising a conductive porous rod and a mixture of a first electrode active material and a first electrolyte residing in pores of the first porous rod; (b) providing a porous separator wrapping around the first electrode to form a separator-protected first electrode; (c) providing a second electrode comprising a conductive porous rod having a mixture of a second electrode active material and a second electrolyte residing in pores of the second porous rod; (d) combining the separator-protected first electrode and the second electrode to form a braid or a yarn; and (d) encasing the braid or yarn with a protective sheath; wherein one of the electrodes is a cathode containing sulfur or a sulfur compound as a cathode active material and the battery has a length-to-diameter aspect ratio no less than 5.

FIELD OF THE INVENTION

This invention is directed at a secondary (rechargeable) lithium-sulfurbattery (including Li—S and Li ion-S cells), sodium-sulfur battery(including Na—S and Na ion-S cells), or their combination or hybrid cellthat is flexible, conformal, and non-flammable.

BACKGROUND OF THE INVENTION

Conventional batteries (e.g. 18650-type cylindrical cells, rectangularpouch cells, or prismatic cells) are mechanically rigid and thisnon-flexibility feature has severely constrained its adaptability orfeasibility of being implemented in confined spaces or for use inwearable devices. Flexible and shape-conformable power sources can beused to overcome these design limitations. These new power sources willenable the development of next-generation electronic devices, such assmart mobile gadgets, roll-up displays, wearable devices, and biomedicalsensors. Flexible and conformable power sources will also save weightand space in electric vehicles.

Rechargeable lithium-ion (Li-ion) and lithium metal batteries (includingLi-sulfur and Li metal-air batteries) are considered promising powersources for electric vehicle (EV), hybrid electric vehicle (HEV), andportable electronic devices, such as lap-top computers and mobilephones. Historically, rechargeable lithium metal batteries were producedusing non-lithiated compounds having relatively high specificcapacities, such as TiS₂, MoS₂, MnO₂, CoO₂, and V₂O₅, as the cathodeactive materials, which were coupled with a lithium metal anode. Whenthe battery was discharged, lithium ions were transferred from thelithium metal anode through the electrolyte to the cathode, and thecathode became lithiated. Unfortunately, upon repeatedcharges/discharges, the lithium metal resulted in the formation ofdendrites at the anode that ultimately grew to penetrate through theseparator, causing internal shorting and explosion. As a result of aseries of accidents associated with this problem, the production ofthese types of secondary batteries was stopped in the early 1990's,giving ways to lithium-ion batteries.

In lithium-ion batteries, pure lithium metal sheet or film was replacedby carbonaceous materials as the anode. The carbonaceous materialabsorbs lithium (through intercalation of lithium ions or atoms betweengraphene planes, for instance) and desorbs lithium ions during there-charge and discharge phases, respectively, of the lithium ion batteryoperation. The carbonaceous material may comprise primarily graphitethat can be intercalated with lithium and the resulting graphiteintercalation compound may be expressed as Li_(x)C₆, where x istypically less than 1.

Although lithium-ion (Li-ion) batteries are promising energy storagedevices for electric drive vehicles, state-of-the-art Li-ion batterieshave yet to meet the cost and performance targets. Li-ion cellstypically use a lithium transition-metal oxide or phosphate as apositive electrode (cathode) that de/re-intercalates Li⁺ at a highpotential with respect to the carbon negative electrode (anode). Thespecific capacity of lithium transition-metal oxide or phosphate basedcathode active material is typically in the range of 140-170 mAh/g. As aresult, the specific energy of commercially available Li-ion cells istypically in the range of 120-250 Wh/kg, most typically 150-220 Wh/kg.These specific energy values are two to three times lower than whatwould be required for battery-powered electric vehicles to be widelyaccepted.

With the rapid development of hybrid (HEV), plug-in hybrid electricvehicles (HEV), and all-battery electric vehicles (EV), there is anurgent need for anode and cathode materials that provide a rechargeablebattery with a significantly higher specific energy, higher energydensity, higher rate capability, long cycle life, and safety. One of themost promising energy storage devices is the lithium-sulfur (Li—S) cellsince the theoretical capacity of Li is 3,861 mAh/g and that of S is1,675 mAh/g. In its simplest form, a Li—S cell consists of elementalsulfur as the positive electrode and lithium as the negative electrode.The lithium-sulfur cell operates with a redox couple, described by thereaction S₈+16Li↔8Li₂S that lies near 2.2 V with respect to Li⁺/Liº.This electrochemical potential is approximately ⅔ of that exhibited byconventional positive electrodes (e.g. LiMnO₄) in a conventionallithium-ion battery. However, this shortcoming is offset by the veryhigh theoretical capacities of both Li and S. Thus, compared withconventional intercalation-based Li-ion batteries, Li—S cells have theopportunity to provide a significantly higher energy density (a productof capacity and voltage). Assuming complete reaction to Li₂S, energydensities values can approach 2,500 Wh/kg and 2,800 Wh/L, respectively,based on the combined Li and S weight or volume. If based on the totalcell weight or volume, the energy densities can reach approximately1,000 Wh/kg and 1,300 Wh/L, respectively. However, the current Li-sulfurcells reported by industry leaders in sulfur cathode technology have amaximum cell specific energy of 250-400 Wh/kg and 350-550 Wh/L (based onthe total cell weight or volume), which are far below what is possible.

In summary, despite its considerable advantages, the Li—S cell isplagued with several major technical problems that have thus farhindered its widespread commercialization:

-   (1) Conventional lithium metal cells still have dendrite formation    and related internal shorting issues.-   (2) Sulfur or sulfur-containing organic compounds are highly    insulating, both electrically and ionically. To enable a reversible    electrochemical reaction at high current densities or    charge/discharge rates, the sulfur must maintain intimate contact    with an electrically conductive additive. Various carbon-sulfur    composites have been utilized for this purpose, but only with    limited success owing to the limited scale of the contact area.    Typical reported capacities are between 300 and 550 mAh/g (based on    the cathode carbon-sulfur composite weight) at moderate rates.-   (3) The cell tends to exhibit a rapid and significant capacity decay    during discharge-charge cycling. This is mainly due to the high    solubility of the lithium polysulfide anions formed as reaction    intermediates during both discharge and charge processes in the    polar organic solvents used in electrolytes. During cycling, the    lithium polysulfide anions can migrate through the separator to the    Li negative electrode whereupon they are reduced to solid    precipitates (Li₂S₂ and/or Li₂S), causing active mass loss. In    addition, the solid product that precipitates on the surface of the    positive electrode during discharge becomes electrochemically    irreversible, which also contributes to active mass loss.-   (4) More generally speaking, a significant drawback with cells    containing cathodes comprising elemental sulfur, organosulfur and    carbon-sulfur materials relates to the dissolution and excessive    out-diffusion of soluble sulfides, polysulfides, organo-sulfides,    carbon-sulfides and/or carbon-polysulfides (hereinafter referred to    as anionic reduction products) from the cathode into the rest of the    cell. This phenomenon is commonly referred to as the Shuttle Effect.    This process leads to several problems: high self-discharge rates,    loss of cathode capacity, corrosion of current collectors and    electrical leads leading to loss of electrical contact to active    cell components, fouling of the anode surface giving rise to    malfunction of the anode, and clogging of the pores in the cell    membrane separator which leads to loss of ion transport and large    increases in internal resistance in the cell.

In response to these challenges, new electrolytes, protective films forthe lithium anode, and solid electrolytes have been developed. Someinteresting cathode developments have been reported recently to containlithium polysulfides; but, their performance still fall short of what isrequired for practical applications.

Despite the various approaches proposed for the fabrication of highenergy density Li—S cells, there remains a need for cathode materialsand production processes that improve the utilization of electro-activecathode materials (S utilization efficiency), and provide rechargeableLi—S cells with high capacities over a large number of cycles. Mostsignificantly, lithium metal (including pure lithium, lithium alloys ofhigh lithium content with other metal elements, or lithium-containingcompounds with a high lithium content; e.g. >80% or preferably >90% byweight Li) still provides the highest anode specific capacity ascompared to essentially all other anode active materials (except puresilicon, but silicon has pulverization issues). Lithium metal would bean ideal anode material in a lithium-sulfur secondary battery ifdendrite related issues could be addressed.

Sodium metal (Na) and potassium metal (K) have similar chemicalcharacteristics to Li and the sulfur cathode in room temperaturesodium-sulfur cells (RT Na—S batteries) or potassium-sulfur cells (K—S)face the same issues observed in Li—S batteries, such as: (i) low activematerial utilization rate, (ii) poor cycle life, and (iii) low Coulombicefficiency. Again, these drawbacks arise mainly from insulating natureof S, dissolution of S and Na or K polysulfide intermediates in liquidelectrolytes (and related Shuttle effect), and large volume changeduring charge/discharge.

It may be noted that in most of the open literature reports (scientificpapers) and patent documents, scientists or inventors choose to expressthe cathode specific capacity based on the sulfur or lithium polysulfideweight alone (not the total cathode composite weight), but unfortunatelya large proportion of non-active materials (those not capable of storinglithium, such as conductive additive and binder) is typically used intheir Li—S cells. For practical use purposes, it is more meaningful touse the cathode composite weight-based capacity value.

Low-capacity anode or cathode active materials are not the only problemassociated with the lithium-sulfur or sodium-sulfur battery. There areserious design and manufacturing issues that the battery industry doesnot seem to be aware of, or has largely ignored. For instance, despitethe seemingly high gravimetric capacities at the electrode level (basedon the anode or cathode active material weight alone) as frequentlyclaimed in open literature and patent documents, these electrodesunfortunately fail to provide batteries with high capacities at thebattery cell or pack level (based on the total battery cell weight orpack weight). This is due to the notion that, in these reports, theactual active material mass loadings of the electrodes are too low. Inmost cases, the active material mass loadings of the anode (arealdensity) is significantly lower than 15 mg/cm² and mostly <8 mg/cm²(areal density=the amount of active materials per electrodecross-sectional area along the electrode thickness direction). Thecathode active material amount is typically 1.5-2.5 times higher thanthe anode active material amount in a cell. As a result, the weightproportion of the anode active material (e.g. carbon) in a Na ion-sulfuror Li ion-sulfur battery cell is typically from 15% to 20%, and that ofthe cathode active material from 20% to 35% (mostly <30%). The weightfraction of the cathode and anode active materials combined is typicallyfrom 35% to 50% of the cell weight.

The low active material mass loading is primarily due to the inabilityto obtain thicker electrodes (thicker than 100-200 μm) using theconventional slurry coating procedure. This is not a trivial task as onemight think, and in reality the electrode thickness is not a designparameter that can be arbitrarily and freely varied for the purpose ofoptimizing the cell performance. Contrarily, thicker samples tend tobecome extremely brittle or of poor structural integrity and would alsorequire the use of large amounts of binder resin. Due to the low-meltingand soft characteristics of sulfur, it has been practically impossibleto produce a sulfur cathode thicker than 100 μm. Furthermore, in a realbattery manufacturing facility, a coated electrode thicker than 150 μmwould require a heating zone as long as 100 meters to thoroughly dry thecoated slurry. This would significantly increase the equipment cost andreduce the production throughput. The low areal densities and low volumedensities (related to thin electrodes and poor packing density) resultin a relatively low volumetric capacity and low volumetric energydensity of the battery cells.

With the growing demand for more compact and portable energy storagesystems, there is keen interest to increase the utilization of thevolume of the batteries. Novel electrode materials and designs thatenable high volumetric capacities and high mass loadings are essentialto achieving improved cell volumetric capacities and energy densities.

Thus, an object of the present invention is to provide a rechargeablealkali metal-sulfur cell based on rational materials and battery designsthat overcome or significantly reduce the following issues commonlyassociated with conventional Li—S and Na—S cells: (a) dendrite formation(internal shorting); (b) extremely low electric and ionic conductivitiesof sulfur, requiring large proportion (typically 30-55%) of non-activeconductive fillers and having significant proportion of non-accessibleor non-reachable sulfur or alkali metal polysulfides); (c) dissolutionof S and alkali metal polysulfide in electrolyte and migration ofpolysulfides from the cathode to the anode (which irreversibly reactwith Li or Na metal at the anode), resulting in active material loss andcapacity decay (the shuttle effect); (d) short cycle life; and (e) lowactive mass loading in both the anode and the cathode.

A specific object of the present invention is to provide a rechargeablealkali metal-sulfur battery (e.g. mainly Li—S and room temperature Na—Sbattery) that exhibits an exceptionally high specific energy or highenergy density. One particular technical goal of the present inventionis to provide an alkali metal-sulfur or alkali ion-sulfur cell with acell specific energy greater than 400 Wh/Kg, preferably greater than 500Wh/Kg, more preferably greater than 600 Wh/Kg, and most preferablygreater than 700 Wh/kg (all based on the total cell weight). Preferably,the volumetric energy density is greater than 600 Wh/L, furtherpreferably greater than 800 Wh/L, and most preferably greater than 1,000Wh/L.

Another object of the present invention is to provide an alkalimetal-sulfur cell that exhibits a high cathode specific capacity, higherthan 1,200 mAh/g based on the sulfur weight, or higher than 1,000 mAh/gbased on the cathode composite weight (including sulfur, conductingadditive or substrate, and binder weights combined, but excluding theweight of cathode current collector). The specific capacity ispreferably higher than 1,400 mAh/g based on the sulfur weight alone orhigher than 1,200 mAh/g based on the cathode composite weight. This mustbe accompanied by a high specific energy, good resistance to dendriteformation, and a long and stable cycle life.

Additionally, thick electrodes are also mechanically rigid, notflexible, not bendable, and not conformal to a desired shape. As such,for conventional alkali metal-sulfur batteries, highvolumetric/gravimetric energy density and mechanical flexibility appearto be mutually exclusive.

With the growing demand for more compact and portable energy storagesystems, there is keen interest to increase the utilization of thevolume of the batteries. Novel electrode materials and designs thatenable high volumetric capacities and high mass loadings are essentialto achieving improved cell volumetric capacities and energy densitiesfor alkali metal-sulfur batteries.

Therefore, there is clear and urgent need for alkali metal-sulfurbatteries that have high active material mass loading (high arealdensity), active materials with high apparent density (high tapdensity), high electrode thickness without significantly decreasing theelectron and ion transport rates (e.g. without a high electron transportresistance or long lithium or sodium ion diffusion path), highvolumetric capacity, and high volumetric energy density. Theseattributes must be achieved, along with improved flexibility, shapeconformability, and safety of the resulting battery.

SUMMARY OF THE INVENTION

The present invention provides a rope-shaped alkali metal-sulfur batterywherein the alkali metal is selected from Li, Na, or a combinationthereof. The electrodes in the battery are each of a rod or filamentaryshape and porous, containing a mixture of an active material and anelectrolyte pre-impregnated into pores. The filamentary electrode isencased in a porous separator, which is permeable to ions of Li⁺, Na⁺,or K⁺. Multiple electrode filaments are braided or interlaced into abraid or twist yarn, which is encased in a protective sheath. Thebattery has a rope shape having a length-to-diameter orlength-to-thickness aspect ratio no less than 5, preferably no less than10, more preferably greater than 15, further more preferably greaterthan 20, still more preferably greater than 50. There is no theoreticallimit on the aspect ratio. However, the cable length can be miles longand, with a cable diameter of 5 mm and a length of >10 km, a practicalaspect ratio limit may be >10⁶.

In some embodiments, the battery comprises: (a) a first electrodecomprising a first electrically conductive porous rod or filament havingpores and a first mixture of a first electrode active material and afirst electrolyte, wherein the first mixture resides in the pores of thefirst porous rod or filament; (b) a porous separator wrapping around orencasing the first electrode to form a separator-protected firstelectrode; (c) a second electrode comprising a second electricallyconductive porous rod or filament having pores and a second mixture of asecond electrode active material and a second electrolyte, wherein thesecond mixture resides in the pores of the second porous rod orfilament; wherein the separator-protected first electrode and the secondelectrode are combined or interlaced together to form a braid or a yarnhaving a twist or spiral electrode and the first electrode and secondelectrode contains an anode and a cathode; and (d) a protective casingor sheath wrapping around or encasing the braid or yarn; wherein eitherthe first electrode or the second electrode is a cathode containingsulfur or a sulfur compound as a cathode active material and the batteryhas a rope shape having a length-to-diameter or length-to-thicknessaspect ratio no less than 5.

In this battery structure, either the first electrode or the secondelectrode is a cathode and either the first electrode active material orthe second electrode active material is a cathode active material thatcontains sulfur or a sulfur compound (e.g. organo-sulfur,polymer-sulfur, carbon-sulfur, metal sulfide, S—Sb, S—Bi, S—Se, S—Temixture materials, etc.) as a cathode active material. The firstelectrolyte can be the same or different from the second electrolyte.

The invention provides a process for producing a rope-shape alkalimetal-sulfur battery, comprising (a) providing a first electrodecomprising a conductive porous rod and a mixture of a first electrodeactive material and a first electrolyte residing in pores of the firstporous rod; (b) providing a porous separator wrapping around the firstelectrode to form a separator-protected first electrode; (c) providing asecond electrode comprising a conductive porous rod having a mixture ofa second electrode active material and a second electrolyte residing inpores of the second porous rod; (d) combining the separator-protectedfirst electrode and the second electrode to form a braid or a yarn; and(d) encasing the braid or yarn with a protective sheath; wherein one ofthe electrodes is a cathode containing sulfur or a sulfur compound as acathode active material and the battery has a length-to-diameter aspectratio no less than 5.

In certain preferred embodiments, the cathode active material isselected from (A) sulfur bonded to pore walls of the porous rod orfilament, (B) sulfur bonded to or confined by a carbon or graphitematerial, (C) sulfur bonded to or confined by a polymer, (D)sulfur-carbon compound, (E) metal sulfide M_(x)S_(y), wherein x is aninteger from 1 to 3 and y is an integer from 1 to 10, and M is a metalelement selected from Li, Na, K, Mg, Ca, a transition metal, a metalfrom groups 13 to 17 of the periodic table, or a combination thereof.

In this alkali metal-sulfur battery, the first electrode can be anegative electrode (or anode) and the second electrode a positiveelectrode (or cathode); or vice versa. Multiple filamentary anodes maybe combined with one or multiple filamentary cathodes to form a braid ortwist yarn. Multiple filamentary cathodes may be combined with one ormultiple filamentary anodes to form a braid or twist yarn.

The electrically conductive porous rod (or filament) in the first orsecond electrode may contain a porous foam selected from a metal foam,metal web, metal fiber mat, metal nanowire mat, conductive polymer fibermat, conductive polymer foam, conductive polymer-coated fiber foam,carbon foam, graphite foam, carbon aerogel, graphene aerogel, carbonxerogel, graphene foam, graphene oxide foam, reduced graphene oxidefoam, carbon fiber foam, graphite fiber foam, exfoliated graphite foam,or a combination thereof. These foams can be made into highly deformableand conformable structures. The electrically conductive porous rod maycontain a carbon/graphite fiber, fiber tow, fiber yarn, fiber braid, orfiber knit structure that is porous.

The aforementioned foam structures can be readily made into a porositylevel >50%, more typically >70%, more typically and preferably >80%,still more typically and preferably >90%, and most preferably >95%(graphene aerogel can exceed a 99% porosity level). The skeletonstructure (pore walls) in these foams forms a 3D network ofelectron-conducting pathways while the pores can accommodate a largeproportion of an electrode active material (anode active material in theanode or cathode active material in the cathode) without using anyconductive additive or a binder resin.

The filamentary or rod-like foam can have a cross-section that iscircular, elliptic, rectangular, square, hexagon, hollow, or irregularin shape. There is no particular restriction on the cross-sectionalshape of the foam structure. The battery has a cable shape that has alength and a diameter or thickness and an aspect ratio (length/thicknessor length/diameter ratio) greater than 10, preferably greater than 15,more preferably greater than 20, further preferably greater than 30,even more preferably greater than 50 or 100. There is no restriction onthe length or diameter (or thickness) of the cable battery. Thethickness or diameter is typically and preferably from 100 nm to 10 cm,more preferably and typically from 1 μm to 1 cm, and most typically from10 μm to 1 mm. The length can run from 1 μm to tens of meters or evenhundreds of meters (if so desired).

In certain embodiments, the invention includes a process for producing arope-shaped alkali metal-sulfur battery wherein the alkali metal isselected from Li, Na, or a combination thereof. The process comprises:(a) providing a first electrode comprising a first electricallyconductive rod and a first mixture of a first electrode active materialand a first electrolyte, wherein the first mixture is deposited on or inthe first rod; (b) wrapping or encasing a porous separator around thefirst electrode to form a separator-protected first electrode; (c)providing a second electrode comprising a second electrically conductiveporous rod having pores and a second mixture of a second electrodeactive material and a second electrolyte, wherein the second mixtureresides in the pores of the second porous rod; (d) combining theseparator-protected first electrode and the second electrode in a twistor spiral manner to form a braid or yarn; and (d) wrapping or encasing aprotective casing or sheath around the braid or yarn to form therope-shape battery. Either the first electrode or the second electrodeis a cathode and either the first electrode active material or thesecond electrode active material is a cathode active material selectedpreferably from sulfur bonded to pore walls of the porous rod, sulfurbonded to or confined by a carbon or graphite material, sulfur bonded toor confined by a polymer, sulfur-carbon compound, metal sulfideM_(x)S_(y), wherein x is an integer from 1 to 3 and y is an integer from1 to 10, and M is a metal element selected from Li, Na, K, Mg, Ca, atransition metal, a metal from groups 13 to 17 of the periodic table, ora combination thereof. The battery has a rope shape having alength-to-diameter or length-to-thickness aspect ratio no less than 5.

In some embodiments, the invention includes a process for producing arope-shaped alkali metal-sulfur battery wherein the alkali metal isselected from Li, Na, or a combination thereof. The process comprises:(a) providing a first electrode comprising an electrically conductiveporous rod having at least 50% by volume of pores and a first mixture ofa first electrode active material and a first electrolyte residing inthe pores of the porous rod; (b) wrapping a porous separator around thefirst electrode to form a separator-protected first electrode; (c)providing a second electrode comprising an electrically conductive rodhaving a second mixture of a second electrode active material and asecond electrolyte deposited thereon or therein; (d) combining orinterlacing the separator-protected first electrode and the secondelectrode in a twist or spiral manner to form a braid or yarn; and (d)wrapping or encasing a protective casing or sheath around the braid oryarn to form the rope-shape battery. Either the first electrode or thesecond electrode is a cathode and either the first electrode activematerial or the second electrode active material is a cathode activematerial selected from sulfur or a sulfur compound. The sulfur compoundmay be selected from organo-sulfur, polymer-sulfur, carbon-sulfur, metalsulfide, S—Sb, S—Bi, S—Se, S—Te mixture, or a combination thereof. Thebattery has a cable shape having a length-to-diameter orlength-to-thickness aspect ratio no less than 5.

In certain embodiments, the rope-shaped battery has a first end and asecond end and the first electrode contains a first terminal connectorcomprising at least one metallic wire, conductive carbon/graphite fiber,or conductive polymer fiber that is embedded in, connected to, orintegral with the first electrode. In certain preferred embodiments, theat least one metallic wire, conductive carbon/graphite fiber, orconductive polymer fiber runs approximately from the first end to thesecond end. This wire or fiber preferably is protruded out of the firstend or second end to become a terminal tab for connecting to anelectronic device or external circuit or load.

Alternatively or additionally, the rope-shaped battery has a first endand a second end and the second electrode contains a second terminalconnector comprising at least one metallic wire, conductivecarbon/graphite fiber, or conductive polymer fiber that is embedded in,connected to, or integral with the second electrode. In certainembodiments, at least one metallic wire, conductive carbon/graphitefiber, or conductive polymer fiber runs approximately from said firstend to said second end. This wire or fiber preferably is protruded outof the first end or second end to become a terminal tab for connectingto an electronic device or external circuit or load.

In some embodiments, the first electrode or second electrode containsparticles, foil, or coating of Li, Na, K, or a combination thereof as anelectrode active material.

In certain embodiments of the invention, the alkali metal battery is alithium-ion battery and the first or second electrode active material isselected from the group consisting of: (a) Particles of naturalgraphite, artificial graphite, meso-carbon microbeads (MCMB), andcarbon; (b) Silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony(Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni), cobalt (Co),manganese (Mn), titanium (Ti), iron (Fe), and cadmium (Cd); (c) Alloysor intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd withother elements, wherein said alloys or compounds are stoichiometric ornon-stoichiometric; (d) Oxides, carbides, nitrides, sulfides,phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites; (e)Pre-lithiated versions thereof; (f) Pre-lithiated graphene sheets; andcombinations thereof.

The pre-lithiated graphene sheets may be selected from pre-lithiatedversions of pristine graphene, graphene oxide, reduced graphene oxide,graphene fluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, boron-doped graphene,nitrogen-doped graphene, chemically functionalized graphene, aphysically or chemically activated or etched version thereof, or acombination thereof.

In some embodiments, the alkali metal-sulfur battery is a sodium-ionsulfur battery and the first or second electrode active materialcontains an alkali intercalation compound selected from petroleum coke,carbon black, amorphous carbon, activated carbon, hard carbon, softcarbon, templated carbon, hollow carbon nanowires, hollow carbon sphere,titanates, NaTi₂(PO₄)₃, Na₂Ti₃O₇, Na₂C₈H₄O₄, Na₂TP, Na_(x)TiO₂ (x=0.2 to1.0), Na₂C₈H₄O₄, carboxylate based materials, C₈H₄Na₂O₄, C₈H₆O₄,C₈H₅NaO₄, C₈Na₂F₄O₄, C₁₀H₂Na₄O₈, C₁₄H₄O₆, C₁₄H₄Na₄O₈, or a combinationthereof.

In some embodiments, the alkali metal-sulfur battery is a sodium-ionsulfur battery and the first or second electrode active materialcontains an alkali intercalation compound selected from the followinggroups of materials: (a) Sodium- or potassium-doped silicon (Si),germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc(Zn), aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni), manganese(Mn), cadmium (Cd), and mixtures thereof; (b) Sodium- orpotassium-containing alloys or intermetallic compounds of Si, Ge, Sn,Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c) Sodium-or potassium-containing oxides, carbides, nitrides, sulfides,phosphides, selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof;(d) Sodium or potassium salts; and (e) Graphene sheets pre-loaded withsodium or potassium on their surfaces; and combinations thereof.

The first electrolyte and/or the second electrolyte may contain alithium salt or sodium salt dissolved in a liquid solvent and whereinthe liquid solvent is water, an organic solvent, an ionic liquid, or amixture of an organic solvent and an ionic liquid. The liquid solventmay be mixed with a polymer to form a polymer gel.

The first electrolyte and/or second electrolyte preferably contains alithium salt or sodium salt dissolved in a liquid solvent having a saltconcentration greater than 2.5 M (preferably >3.0 M, furtherpreferably >3.5 M, even more preferably >5.0 M, still morepreferably >7.0 M, and most preferably >10 M, but typically no greaterthan 15 M).

In the alkali metal-sulfur battery, the electrically conductive porousrod in the first electrode or the electrically conductive porous layerin the second electrode has at least 90% by volume of pores, the firstor second electrode has a diameter or thickness no less than 200 μm orhas an active mass loading occupying at least 30% by weight or by volumeof the entire battery cell, or the first and second electrode activematerials combined occupies at least 50% by weight or by volume of theentire battery cell.

In some preferred embodiments, the electrically conductive porous rod inthe first electrode or the electrically conductive porous layer in thesecond electrode has at least 95% by volume of pores, the first orsecond electrode has a diameter or thickness no less than 300 μm or hasan active mass loading occupying at least 35% by weight or by volume ofthe entire battery cell, or the first and second electrode activematerials combined occupies at least 60% by weight or by volume of theentire battery cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) schematic of a prior art lithium-ion battery cell (as anexample of an alkali metal battery) composed of an anode currentcollector, an anode electrode (e.g. thin Si coating layer), a porousseparator, a cathode electrode (e.g. sulfur layer), and a cathodecurrent collector;

FIG. 1(B) schematic of a prior art lithium-ion battery, wherein theelectrode layer is composed of discrete particles of an active material(e.g. graphite or tin oxide particles in the anode layer or LiCoO₂ inthe cathode layer);

FIG. 1(C) Schematic of a process for producing a rope-shaped flexibleand shape-conformable alkali metal-sulfur battery;

FIG. 1(D) Four examples of the procedure for producing an electrode(anode or cathode) in a continuous and automated manner; and

FIG. 1(E) Schematic of a presently invented process for continuouslyproducing an alkali metal-sulfur battery electrode.

FIG. 2 An electron microscopic image of isolated graphene sheets.

FIG. 3(A) Examples of conductive porous rods: metal grid/mesh and carbonnano-fiber mat.

FIG. 3(B) Examples of conductive porous rods: graphene foam and carbonfoam.

FIG. 3(C) Examples of conductive porous rods: graphite foam and Ni foam.

FIG. 3(D) Examples of conductive porous rods: Cu foam and stainlesssteel foam.

FIG. 4(A) Schematic of a commonly used process for producing exfoliatedgraphite, expanded graphite flakes (thickness >100 nm), and graphenesheets (thickness <100 nm, more typically <10 nm, and can be as thin as0.34 nm).

FIG. 4 (B) Schematic drawing to illustrate the processes for producingexfoliated graphite, expanded graphite flakes, and isolated graphenesheets.

FIG. 5 Ragone plots (gravimetric and volumetric power density vs. energydensity) of Na ion-sulfur battery cells, containing hard carbonparticles as the anode active material and sodium polysulfide particlesas the cathode active materials (along with electrolyte) residing inpores of graphene foam. Two of the 4 data curves are for the cellsprepared according to an embodiment of instant invention (rope-shapecells) and the other two by the conventional slurry coating ofelectrodes (roll-coating).

FIG. 6 Ragone plots (both gravimetric and volumetric power density vs.gravimetric and volumetric energy density) of two Na—S cells, bothcontaining graphene-embraced Na nano particles as the anode activematerial and sulfur coated on graphene pore walls as the cathode activematerial. The data are for both sodium ion cells prepared by thepresently invented method (rope cells) and those by the conventionalslurry coating of electrodes.

FIG. 7 Ragone plots of Li—S batteries containing a lithium foil as theanode active material, sulfur electrochemically deposited in pores ofgraphite foam as the cathode active material, and lithium salt(LiPF₆)—PC/DEC as organic liquid electrolyte. The data are for bothlithium metal-sulfur cells prepared by the presently invented method(rope cells) and those by the conventional slurry coating of electrodes.

FIG. 8 Ragone plot of a series of Li ion-S cells (graphene-wrapped Sinano particles) prepared by the conventional slurry coating process andthe Ragone plot of corresponding rope-shape cells prepared by thepresently invented process.

FIG. 9 The cell-level gravimetric (Wh/kg) and volumetric energydensities (Wh/L) of Li ion-S cell (Pre-lithiated graphiteanode+graphene-supported S cathode) plotted over the achievable cathodethickness range of the S/RGO cathode prepared via the conventionalmethod without delamination and cracking and those by the presentlyinvented method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is directed at a flexible and shape-conformable rope-likealkali metal-sulfur battery exhibiting an exceptionally high volumetricenergy density and high gravimetric energy density. This does notinclude the so-called high-temperature Na—S cell that must operate at atemperature higher than the melting point of the electrolyte(typically >350° C.) and higher than the melting point of sulfur. Theinvented alkali metal-sulfur battery can be a primary battery, but ispreferably a secondary battery selected from an alkali metal-ion battery(e.g. using a Li or Na intercalation compound, such as hard carbonparticles) or an alkali metal-sulfur secondary battery (e.g. using Na orLi metal foil as an anode active material). The battery is based on anaqueous electrolyte, a non-aqueous or organic electrolyte, a gelelectrolyte, an ionic liquid electrolyte, or a mixture of organic andionic liquid. A polymer can be added to these electrolytes to form agel. The electrolyte does not include the solid-state electrolyte.

As illustrated in FIG. 1(A) and FIG. 1(B), a conventional lithium-ion,sodium-ion, Li—S, or Na—S battery cell is typically composed of an anodecurrent collector (e.g. Cu foil), an anode electrode (anode activematerial layer), a porous separator and/or an electrolyte component, acathode electrode (cathode active material layer), and a cathode currentcollector (e.g. Al foil). In a more commonly used cell configuration(FIG. 1(B)), the anode layer is composed of particles of an anode activematerial (e.g. hard carbon particles), a conductive additive (e.g.expanded graphite flakes), and a resin binder (e.g. SBR or PVDF). Thecathode layer is composed of particles of a cathode active material(e.g. NaFePO₄ particles in a Na-ion cell or S-carbon composite particlesin a Li—S cell), a conductive additive (e.g. carbon black particles),and a resin binder (e.g. PVDF). Both the anode and the cathode layersare typically 60-100 μm thick (typically significantly thinner than 200μm) to give rise to a presumably sufficient amount of current per unitelectrode area. Using an active material layer thickness of 100 μm andthe solid (Cu or Al foil) current collector layer thickness of 10 μm asexamples, the resulting battery configuration has a current collectorthickness-to-active material layer thickness ratio of 10/100 or 1/10 forconventional battery cells.

This thickness range of 60-100 μm is considered an industry-acceptedconstraint under which a battery designer normally works under, based onthe current slurry coating process (roll coating of activematerial-binder-additive mixture slurry). This thickness constraint isdue to several reasons: (a) the existing battery electrode coatingmachines are not equipped to coat excessively thin or excessively thickelectrode layers; (b) a thinner layer is preferred based on theconsideration of reduced lithium ion diffusion path lengths; but, toothin a layer (e.g. <60 μm) does not contain a sufficient amount of anactive alkali metal ion storage material (hence, insufficient currentoutput); (c) thicker electrodes are prone to delaminate or crack upondrying or handling after roll-coating of slurry; and (d) thicker coatingrequires an excessively long heating zone (it is not unusual to have aheating zone longer than 100 meters, making the manufacturing equipmentvery expensive). This constraint has made it impossible to freelyincrease the amount of active materials (those responsible for storingNa or Li ions) without increasing the amounts of all non-activematerials (e.g. current collectors and separator) in order to obtain aminimum overhead weight and a maximum sodium storage capability and,hence, a maximized energy density (Wk/kg or Wh/L of cell).

In a less commonly used cell configuration, as illustrated in FIG. 1(A),either the anode active material (e.g. NaTi₂(PO₄)₃ or Na film) or thecathode active material (e.g. lithium transition metal oxide in a Li-ioncell or sulfur/carbon mixture in a Li—S cell) is deposited in a thinfilm form directly onto a current collector, such as a sheet of copperfoil or Al foil using sputtering. However, such a thin film structurewith an extremely small thickness-direction dimension (typically muchsmaller than 500 nm, often necessarily thinner than 100 nm) implies thatonly a small amount of active material can be incorporated in anelectrode (given the same electrode or current collector surface area),providing a low total Na or Li storage capacity per unit electrodesurface area. Such a thin film must have a thickness less than 100 nm tobe more resistant to cycling-induced cracking (for the anode) or tofacilitate a full utilization of the cathode active material. Such aconstraint further diminishes the total Na or Li storage capacity andthe sodium or lithium storage capacity per unit electrode surface area.Such a thin-film battery has very limited scope of application.

On the anode side, a sputtered NaTi₂(PO₄)₃ layer thicker than 100 nm hasbeen found to exhibit poor cracking resistance during batterycharge/discharge cycles. It takes but a few cycles to get fragmented. Onthe cathode side, a layer of sulfur thicker than 100 nm does not allowlithium or sodium ions to fully penetrate and reach full body of thecathode layer, resulting in a poor cathode active material utilizationrate. A desirable electrode thickness is at least 100 μm (not 100 nm),with individual active material particle having a dimension desirablyless than 100 nm. Thus, these thin-film electrodes (with a thickness<100 nm) directly deposited on a current collector fall short of therequired thickness by three (3) orders of magnitude. As a furtherproblem, all of the cathode active materials are not very conductive toboth electrons and sodium/lithium ions. A large layer thickness impliesan excessively high internal resistance and a poor active materialutilization rate.

In other words, there are several conflicting factors that must beconsidered concurrently when it comes to the design and selection of acathode or anode active material in terms of material type, size,electrode layer thickness, and active material mass loading. Thus far,there has been no effective solution offered by any prior art teachingto these often conflicting problems. We have solved these challengingissues, which have troubled battery designers and electrochemists alikefor more than 30 years, by developing a new process of producing alkalimetal-sulfur batteries as herein disclosed.

The prior art sodium or lithium battery cell, including Li—S and roomtemperature Na—S cell, is typically made by a process that includes thefollowing steps: (a) The first step is mixing particles of the anodeactive material (e.g. hard carbon particles), a conductive filler (e.g.expanded graphite flakes), a resin binder (e.g. PVDF) in a solvent (e.g.NMP) to form an anode slurry. On a separate basis, particles of thecathode active material (e.g. sodium metal phosphate particles for theNa-ion cell and LFP particles for the Li-ion cell), a conductive filler(e.g. acetylene black), a resin binder (e.g. PVDF) are mixed anddispersed in a solvent (e.g. NMP) to form a cathode slurry. (b) Thesecond step includes coating the anode slurry onto one or both primarysurfaces of an anode current collector (e.g. Cu foil), drying the coatedlayer by vaporizing the solvent (e.g. NMP) to form a dried anodeelectrode coated on Cu foil. Similarly, the cathode slurry is coated anddried to form a dried cathode electrode coated on Al foil. Slurrycoating is normally done in a roll-to-roll manner in a realmanufacturing situation; (c) The third step includes laminating ananode/Cu foil sheet, a porous separator layer, and a cathode/Al foilsheet together to form a 3-layer or 5-layer assembly, which is cut andslit into desired sizes and stacked to form a rectangular structure (asan example of shape) or rolled into a cylindrical cell structure. (d)The rectangular or cylindrical laminated structure is then encased in analuminum-plastic laminated envelope or steel casing. (e) A liquidelectrolyte is then injected into the laminated structure to make asodium-ion or lithium battery cell.

There are several serious problems associated with the process and theresulting sodium-ion cells and lithium-ion battery cells (or Li—S andNa—S cells):

-   -   1) It is very difficult to produce an electrode layer (anode        layer or cathode layer) that is thicker than 100 μm, let alone        200 μm. There are several reasons why this is the case. An        electrode of 100 μm thickness typically requires a heating zone        of 30-100 meters long in a slurry coating facility, which is too        time consuming, too energy intensive, and not cost-effective.        For some electrode active materials, such as metal oxide        particles or sulfur, it has not been possible to produce an        electrode of good structural integrity that is thicker than 100        μm in a real manufacturing environment on a continuous basis.        The resulting electrodes are very fragile and brittle. Thicker        electrodes have a high tendency to delaminate and crack.    -   2) With a conventional process, as depicted in FIG. 1(A), the        actual mass loadings of the electrodes and the apparent        densities for the active materials are too low to achieve a high        energy density. In most cases, the anode active material mass        loading of the electrodes (areal density) is significantly lower        than 15 mg/cm² and the apparent volume density or tap density of        the active material is typically less than 1.2 g/cm³ even for        relatively large particles of graphite. The cathode active        material mass loading of the electrodes (areal density) is        significantly lower than 10 mg/cm² for the sulfur cathode. In        addition, there are so many other non-active materials (e.g.        conductive additive and resin binder) that add additional        weights and volumes to the electrode without contributing to the        cell capacity. These low areal densities and low volume        densities result in a relatively low gravimetric energy density        and low volumetric energy density.    -   3) The conventional process requires dispersing electrode active        materials (anode active material and cathode active material) in        a liquid solvent (e.g. NMP) to make a slurry and, upon coating        on a current collector surface, the liquid solvent has to be        removed to dry the electrode layer. Once the anode and cathode        layers, along with a separator layer, are laminated together and        packaged in a housing to make a supercapacitor cell, one then        injects a liquid electrolyte (using a salt dissolved in a        solvent different than NMP) into the cell. In actuality, one        makes the two electrodes wet, then makes the electrodes dry, and        finally makes them wet again. Such a wet-dry-wet process is not        a good process at all. Furthermore, the most commonly used        solvent (NMP) is a notoriously undesirable solvent (known to        cause birth defect, for instance).    -   4) Current Li—S and Na—S batteries still suffer from a        relatively low gravimetric energy density and low volumetric        energy density. Hence, neither the Li—S nor room temperature        Na—S battery has made it to the market place.        -   In literature, the energy density data reported based on            either the active material weight alone or the electrode            weight cannot directly translate into the energy densities            of a practical battery cell or device. The “overhead weight”            or weights of other device components (binder, conductive            additive, current collectors, separator, electrolyte, and            packaging) must also be taken into account. The convention            production process results in the weight proportion of the            anode active material (e.g. carbon particles) in a            sodium-ion battery being typically from 15% to 20%, and that            of the cathode active material (e.g. sodium transition metal            oxide) from 20% to 30%.

The present invention provides a process for producing a flexible,shape-conformable, and non-flammable alkali metal-sulfur battery cellhaving a rope shape, high active material mass loading, low overheadweight and volume, high gravimetric energy density, and high volumetricenergy density. In addition, the manufacturing costs of the alkali metalbatteries produced by the presently invented process can besignificantly lower than those by conventional processes since noexpensive slurry coating facilities featuring expensive coating headsand long drying zones are needed.

In one embodiment of the present invention, as illustrated in FIG. 1(C),the present rope-shaped alkali metal battery contains braid- oryarn-shape electrodes. The battery can be made by a process thatincludes a first step of supplying a first electrode 11, which iscomposed of an electrically conductive porous rod having pores that arepartially or fully loaded with a mixture of a first electrode activematerial and a first electrolyte. A conductive additive or a resinbinder may be optionally added into the mixture, but this is notrequired or even desired. This first electrode 11 can optionally containan active material-free and electrolyte-free end section 13 that canserve as a terminal tab for connecting to an external load. This firstelectrode can assume a cross-section that is of any shape; e.g.circular, rectangular, elliptic, square, hexagonal, hollow, or irregularin shape.

Alternatively, in the first step, the first electrode comprises aconductive rod (not a porous foam) and the first mixture is coated ordeposited on the surface of this conductive rod. This rod can be assimple as a metal wire, conductive polymer fiber or yarn, carbon orgraphite fiber or yarn, or multiple thin wires, fibers, or yarns.However, in this situation, the second electrode must contain a porousfoam structure.

The second step involves wrapping around or encasing the first electrode11 with a thin layer of porous separator 15 (e.g. porous plastic film,paper, fiber mat, non-woven, glass fiber cloth, etc.) that is permeableto Li⁺, Na⁺, or K⁺ ions. This step can be as simple as wrapping thefirst electrode with a thin, porous plastic tape in one full circle orslightly more than one full circle, or in a spiral manner. The mainpurpose is to electronically separate the anode and the cathode toprevent internal shorting. The porous separator layer can be simplydeposited all around the first electrode by spraying, printing, coating,dip casting, etc.

The third step involves preparing a second electrode 17 that comprises amixture of a second active material and second electrolyte and,optionally, a conductive additive or resin binder (although notnecessary and not desirable). This second electrode 17 can optionallycontain an active material-free and electrolyte-free end section thatcan serve as a terminal tab for connecting to an external load. Thesecond electrode may be optionally but desirably encased or wrappedaround by a porous separator layer 18.

This second electrode, with or without an encasing porous separatorlayer is then combined with the first electrode using a braiding oryarn-making procedure to make a 2-ply twist yarn or braid. If the firstelectrode is an anode, then the second electrode is a cathode; or viceversa. A yarn or braid can contain multiple anodes (i.e. multiplefilaments or rods each containing an anode active material and anelectrolyte) combined with one single cathode or multiple cathodes. Ayarn or braid can contain multiple cathodes (i.e. multiple filaments orrods each containing a cathode active material and an electrolyte)combined with one single anode or multiple anode filaments. As the finalstep, this braid or yarn structure is encased or protected by aprotective casing or sheath 19 that is electrically insulating (e.g. aplastic sheath or rubber shell).

It may be noted that some additional electrolyte may be incorporatedbetween the n-ply braid/yarn (n≥2) and the protective sheath. However,this is not a requirement since all the electrode rods or filamentsalready contain an active material and an electrolyte in their pores.

In some embodiments, one of the electrodes comprises a porous rod havingpores to accommodate an active material-electrolyte mixture and at leastone of the electrodes is a non-porous rod (filament, fiber, wire, etc.)having an active material-electrolyte mixture coated on its surface.

There are several means of making the first electrode or the secondelectrode. As schematically illustrated in FIG. 1(D) and using a squareor rectangular foam rod cross-section as an example, one preferredmethod comprises continuously feeding one or a plural of electricallyconductive porous foam rod (e.g. 304, 310, 322, or 330; only one foamrod being shown, but there can be multiple rods in each case), from afeeder roller (not shown), into an active material/electrolyteimpregnation zone where a wet active material mixture (e.g. slurry,suspension, or gel-like mass, such as 306 a, 306 b, 312 a, 312 b) of anelectrode active material, an electrolyte and an optional conductiveadditive is delivered to at least a porous surface of the porous rod(e.g. 304 or 310 in Schematic A and schematic B, respectively, of FIG.1(D)). Using Schematic A as an example, the wet activematerial/electrolyte mixture (306 a, 306 b) is forced to impregnate intothe porous rod from both sides using one or two pairs of rollers (302 a,302 b, 302 c, and 302 d) to form an impregnated active electrode 308 (ananode or cathode). The conductive porous foam rod containsinterconnected network of electron-conducting pathways and at least 50%by volume of pores (preferably >70%, more preferably >80%, furtherpreferably >90%, and most preferably >95%).

In Schematic B, two feeder rollers 316 a, 316 b are used to continuouslypay out two protective films 314 a, 314 b that support wet activematerial/electrolyte mixture layers 312 a, 312 b. These wet activematerial/electrolyte mixture layers 312 a, 312 b can be delivered to theprotective (supporting) films 314 a, 314 b using a broad array ofprocedures (e.g. printing, spraying, casting, coating, etc., which arewell known in the art). As the conductive porous foam rod 110 movesthough the gaps between two sets of rollers (318 a, 318 b, 318 c, 318d), the wet active mixture material/electrolyte is impregnated into thepores of the porous rod 310 to form an active material electrode 320 (ananode or cathode electrode layer) covered by two protective films 314 a,314 b. These protective films can be later removed.

Using Schematic C as another example, two spraying devices 324 a, 324 bwere used to dispense the wet active material/electrolyte mixture (325a, 325 b) to the two opposed porous surfaces of the conductive porousrod 322. The wet active material mixture is forced to impregnate intothe porous rod from both sides using one or two pairs of rollers to forman impregnated active electrode 326 (an anode or cathode). Similarly, inSchematic D, two spraying devices 332 a, 332 b were used to dispense thewet active material mixture (333 a, 333 b) to the two opposed porousfoam rod surfaces of the conductive porous rod 330. The wet activematerial-electrolyte mixture is forced to impregnate into the porous rodfrom both sides using one or two pairs of rollers to form an impregnatedactive electrode 338 (an anode or cathode).

As another example, as illustrated in Schematic E of FIG. 1(E), theelectrode production process begins by continuously feeding a conductiveporous foam rod or filament 356 of any cross-sectional shape from afeeder roller 340. The porous layer 356 is directed by a roller 342 toget immersed into a wet active material mixture mass 346 (slurry,suspension, gel, etc.) in a container 344. The active material mixturebegins to impregnate into pores of the porous rod or filament 356 as ittravels toward roller 342 b and emerges from the container to feed intothe gap between two rollers 348 a, 348 b. Two protective films 350 a,350 b are concurrently fed from two respective rollers 352 a, 352 b tocover the impregnated porous layer 354, which may be continuouslycollected on a rotating drum (a winding roller 355). The process isapplicable to both the anode and the cathode electrodes.

The resulting electrode rod or filament (anode or cathode electrode) canhave a thickness or diameter from 100 nm to several centimeters (orthicker, if so desired). For a micro-cable (e.g. as a flexible powersource for a micro-electronic device) the electrode thickness ordiameter is from 100 nm to 100 μm, more typically from 1 μm to 50 μm,and most typically from 10 μm to 30 μm. For a macroscopic, flexible andconformal cable battery (e.g. for use in confined spaces in an electricvehicle, EV), the electrode typically and desirably has a thickness noless than 100 μm (preferably >200 μm, further preferably >300 μm, morepreferably >400 μm; further more preferably >500 μm, 600 μm, oreven >1,000 μm; no theoretical limitation on the electrode thickness.

The above are but several examples to illustrate how the presentlyinvented flexible and shape-conformable rope-like alkali metal batteriescan be made. These examples should not be used to limit the scope of theinstant invention.

The electrically conductive porous rods or filaments may be selectedfrom metal foam, metal web or screen, perforated metal sheet-basedstructure, metal fiber mat, metal nanowire mat, conductive polymernano-fiber mat, conductive polymer foam, conductive polymer-coated fiberfoam, carbon foam, graphite foam, carbon aerogel, carbon xerogel,graphene aerogel, graphene foam, graphene oxide foam, reduced grapheneoxide foam, carbon fiber foam, graphite fiber foam, exfoliated graphitefoam, or a combination thereof. The porous rods or fikaments must bemade of an electrically conductive material, such as a carbon, graphite,metal, metal-coated fiber, conductive polymer, or conductivepolymer-coated fiber, which is in a form of highly porous mat,screen/grid, non-woven, foam, etc. Examples of conductive porous layersare presented in FIG. 3(A), FIG. 3(B), FIG. 3(C), and FIG. 3(D). Theporosity level must be at least 50% by volume, preferably greater than70%, further preferably greater than 90%, and most preferably greaterthan 95% by volume. The backbone of the foam or the foam walls forms anetwork of electron-conducting pathways.

These foam structures can be readily made into any cross-sectionalshape. They also can be very flexible; typically, non-metallic foamsbeing more flexible than metallic foams. However, metal nano-fibers canbe made into highly flexible foams. Since the electrolyte is in either aliquid or gel state, the resulting cable battery can be very flexibleand can be made to be conformal to essentially any odd shape. Even whenthe salt concentration in a liquid solvent is high (e.g. from 2.5 M to15 M), the foam structure containing electrolyte inside their poresremains deformable, bendable, twistable, and conformable to even an oddshape.

In some embodiments, the electrically conductive porous rod in the firstor second electrode contains a conductive polymer fiber, acarbon/graphite fiber, a fiber tow, fiber yarn, fiber braid, fiber knitstructure that is made of a conductive polymer, carbon, or graphitefiber and is porous.

Preferably, substantially all of the pores in the original conductiveporous rods or filaments are filled with the electrode active material(anode or cathode), electrolyte, and optional conductive additive (nobinder resin needed). Since there are great amounts of pores (moretypically 70-99% or preferably 85%-99%) relative to the pore walls orconductive pathways (1-30%), very little space is wasted (“being wasted”means not being occupied by the electrode active material andelectrolyte), resulting in high proportion of electrode activematerial-electrolyte mixture (high active material loading mass).

In such battery electrode configurations (e.g. FIG. 1(C)), the electronsonly have to travel a short distance (half of the pore size, on average;e.g. nanometers or a few micrometers) before they are collected by thepore walls since pore walls are present everywhere throughout the entireelectrode structure (the conductive foam serving as a currentcollector). These pore walls form a 3-D network of interconnectedelectron-transporting pathways with minimal resistance. Additionally, ineach anode electrode or cathode electrode, all electrode active materialparticles are pre-dispersed in a liquid electrolyte (no wettabilityissue), eliminating the existence of dry pockets commonly present in anelectrode prepared by the conventional process of wet coating, drying,packing, and electrolyte injection. Thus, the presently invented processdelivers a totally unexpected advantage over the conventional batterycell production process.

In a preferred embodiment, the anode active material is a prelithiatedor pre-sodiated version of graphene sheets selected from pristinegraphene, graphene oxide, reduced graphene oxide, graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene, ora combination thereof. The starting graphitic material for producing anyone of the above graphene materials may be selected from naturalgraphite, artificial graphite, meso-phase carbon, meso-phase pitch,meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber,carbon nano-fiber, carbon nano-tube, or a combination thereof. Graphenematerials are also a good conductive additive for both the anode andcathode active materials of an alkali metal battery.

The constituent graphene planes of a graphite crystallite in a naturalor artificial graphite particle can be exfoliated and extracted orisolated to obtain individual graphene sheets of hexagonal carbon atoms,which are single-atom thick, provided the inter-planar van der Waalsforces can be overcome. An isolated, individual graphene plane of carbonatoms is commonly referred to as single-layer graphene. A stack ofmultiple graphene planes bonded through van der Waals forces in thethickness direction with an inter-graphene plane spacing ofapproximately 0.3354 nm is commonly referred to as a multi-layergraphene. A multi-layer graphene platelet has up to 300 layers ofgraphene planes (<100 nm in thickness), but more typically up to 30graphene planes (<10 nm in thickness), even more typically up to 20graphene planes (<7 nm in thickness), and most typically up to 10graphene planes (commonly referred to as few-layer graphene inscientific community). Single-layer graphene and multi-layer graphenesheets are collectively called “nano graphene platelets” (NGPs), asshown in FIG. 2. Graphene sheets/platelets (collectively, NGPs) are anew class of carbon nano material (a 2-D nano carbon) that is distinctfrom the 0-D fullerene, the 1-D CNT or CNF, and the 3-D graphite. Forthe purpose of defining the claims and as is commonly understood in theart, a graphene material (isolated graphene sheets) is not (and does notinclude) a carbon nanotube (CNT) or a carbon nano-fiber (CNF).

In one process, graphene materials are obtained by intercalating naturalgraphite particles with a strong acid and/or an oxidizing agent toobtain a graphite intercalation compound (GIC) or graphite oxide (GO),as illustrated in FIG. 4(A) and FIG. 4(B) (schematic drawings). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes in a GIC or GO serves to increase theinter-graphene spacing (d₀₀₂, as determined by X-ray diffraction),thereby significantly reducing the van der Waals forces that otherwisehold graphene planes together along the c-axis direction. The GIC or GOis most often produced by immersing natural graphite powder (100 in FIG.4(B)) in a mixture of sulfuric acid, nitric acid (an oxidizing agent),and another oxidizing agent (e.g. potassium permanganate or sodiumperchlorate). The resulting GIC (102) is actually some type of graphiteoxide (GO) particles if an oxidizing agent is present during theintercalation procedure. This GIC or GO is then repeatedly washed andrinsed in water to remove excess acids, resulting in a graphite oxidesuspension or dispersion, which contains discrete and visuallydiscernible graphite oxide particles dispersed in water. In order toproduce graphene materials, one can follow one of the two processingroutes after this rinsing step, briefly described below:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (106) that typically have athickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,112), as disclosed in our U.S. application Ser. No. 10/858,814 (Jun. 3,2004). Single-layer graphene can be as thin as 0.34 nm, whilemulti-layer graphene can have a thickness up to 100 nm, but moretypically less than 10 nm (commonly referred to as few-layer graphene).Multiple graphene sheets or platelets may be made into a sheet of NGPpaper using a paper-making process. This sheet of NGP paper is anexample of the porous graphene structure layer utilized in the presentlyinvented process.

Route 2 entails ultrasonicating the graphite oxide suspension (e.g.graphite oxide particles dispersed in water) for the purpose ofseparating/isolating individual graphene oxide sheets from graphiteoxide particles. This is based on the notion that the inter-grapheneplane separation bas been increased from 0.3354 nm in natural graphiteto 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakeningthe van der Waals forces that hold neighboring planes together.Ultrasonic power can be sufficient to further separate graphene planesheets to form fully separated, isolated, or discrete graphene oxide(GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% by weightof oxygen.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials.

Pristine graphene, in smaller discrete graphene sheets (typically 0.3 μmto 10 μm), may be produced by direct ultrasonication (also known asliquid phase exfoliation or production) or supercritical fluidexfoliation of graphite particles. These processes are well-known in theart.

The graphene oxide (GO) may be obtained by immersing powders orfilaments of a starting graphitic material (e.g. natural graphitepowder) in an oxidizing liquid medium (e.g. a mixture of sulfuric acid,nitric acid, and potassium permanganate) in a reaction vessel at adesired temperature for a period of time (typically from 0.5 to 96hours, depending upon the nature of the starting material and the typeof oxidizing agent used). As previously described above, the resultinggraphite oxide particles may then be subjected to thermal exfoliation orultrasonic wave-induced exfoliation to produce isolated GO sheets. TheseGO sheets can then be converted into various graphene materials bysubstituting —OH groups with other chemical groups (e.g. —Br, NH₂,etc.).

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished.

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF)_(n)carbon atoms are sp3-hybridized and thus the fluorocarbon layers arecorrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n)only half of the C atoms are fluorinated and every pair of the adjacentcarbon sheets are linked together by covalent C—C bonds. Systematicstudies on the fluorination reaction showed that the resulting F/C ratiois largely dependent on the fluorination temperature, the partialpressure of the fluorine in the fluorinating gas, and physicalcharacteristics of the graphite precursor, including the degree ofgraphitization, particle size, and specific surface area. In addition tofluorine (F₂), other fluorinating agents may be used, although most ofthe available literature involves fluorination with F₂ gas, sometimes inpresence of fluorides.

For exfoliating a layered precursor material to the state of individuallayers or few-layers, it is necessary to overcome the attractive forcesbetween adjacent layers and to further stabilize the layers. This may beachieved by either covalent modification of the graphene surface byfunctional groups or by non-covalent modification using specificsolvents, surfactants, polymers, or donor-acceptor aromatic molecules.The process of liquid phase exfoliation includes ultrasonic treatment ofa graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 4(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 4(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as graphite worms 104. These wormsof graphite flakes which have been greatly expanded can be formedwithout the use of a binder into cohesive or integrated sheets ofexpanded graphite, e.g. webs, papers, strips, tapes, foils, mats or thelike (typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

Acids, such as sulfuric acid, are not the only type of intercalatingagent (intercalant) that penetrate into spaces between graphene planesto obtain GICs. Many other types of intercalating agents, such as alkalimetals (Li, K, Na, Cs, and their alloys or eutectics), can be used tointercalate graphite to stage 1, stage 2, stage 3, etc. Stage n impliesone intercalant layer for every n graphene planes. For instance, astage-1 potassium-intercalated GIC means there is one layer of K forevery graphene plane; or, one can find one layer of K atoms insertedbetween two adjacent graphene planes in a G/K/G/K/G/KG . . . sequence,where G is a graphene plane and K is a potassium atom plane. A stage-2GIC will have a sequence of GG/K/GG/K/GG/K/GG and a stage-3 GIC willhave a sequence of GGG/K/GGG/K/GGG . . . , etc. These GICs can then bebrought in contact with water or water-alcohol mixture to produceexfoliated graphite and/or separated/isolated graphene sheets.

Exfoliated graphite worms may be subjected to high-intensity mechanicalshearing/separation treatments using a high-intensity air jet mill,high-intensity ball mill, or ultrasonic device to produce separated nanographene platelets (NGPs) with all the graphene platelets thinner than100 nm, mostly thinner than 10 nm, and, in many cases, beingsingle-layer graphene (also illustrated as 112 in FIG. 4(B)). An NGP iscomposed of a graphene sheet or a plurality of graphene sheets with eachsheet being a two-dimensional, hexagonal structure of carbon atoms. Amass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene or graphene oxide may be madeinto a porous graphene film (114 in FIG. 4(B)) using a film-makingprocess. Alternatively, with a low-intensity shearing, graphite wormstend to be separated into the so-called expanded graphite flakes (108 inFIG. 4(B) having a thickness >100 nm. These flakes can be formed intographite mat or nonwoven 106 using mat-making process, with or without aresin binder, to form an expanded graphite foam. Graphite foams can bemade by graphitization of carbon foams as well.

In some embodiments, the first electrode or second electrode containsparticles, foil, or coating of Li, Na, or a combination thereof as anelectrode active material.

There is no particular restriction on the types of anode activematerials that can be used in practicing the instant invention.Preferably, in the invented process, the anode active material absorbsalkali ions (e.g. lithium ions) at an electrochemical potential of lessthan 1.0 volt (preferably less than 0.7 volts) above the Li/Li⁺ (i.e.relative to Li→Li⁺+e⁻ as the standard potential) the NaNa⁺ referencewhen the battery is charged. In one preferred embodiment, the anodeactive material is selected from the group consisting of: (a) Particlesof natural graphite, artificial graphite, meso-carbon microbeads (MCMB),and carbon (including soft carbon, hard carbon, carbon nano-fiber, andcarbon nano-tube); (b) Silicon (Si), germanium (Ge), tin (Sn), lead(Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel(Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), and cadmium(Cd); (Si, Ge, Al, and Sn are most desirable due to their high specificcapacities; (c) Alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, or Cd with other elements, wherein the alloys or compoundsare stoichiometric or non-stoichiometric (e.g. SiAl, SiSn); (d) Oxides,carbides, nitrides, sulfides, phosphides, selenides, and tellurides ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and theirmixtures or composites (e.g. SnO, TiO₂, Co₃O₄, etc.); (e) Pre-lithiatedor pre-sodiated versions thereof (e.g. pre-lithiated TiO₂, which islithium titanate); (f) Pre-lithiated or pre-sodiated graphene sheets;and combinations thereof.

In another preferred embodiment, the anode active material is apre-sodiated or pre-lithiated version of graphene sheets selected frompristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof. The starting graphitic material forproducing any one of the above graphene materials may be selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.Graphene materials are also a good conductive additive for both theanode and cathode active materials of an alkali metal battery.

In the rechargeable alkali metal-sulfur battery, the anode may containan alkali ion source selected from an alkali metal, an alkali metalalloy, a mixture of alkali metal or alkali metal alloy with an alkaliintercalation compound, an alkali element-containing compound, or acombination thereof. Particularly desired is an anode active materialthat contains an alkali intercalation compound selected from petroleumcoke, carbon black, amorphous carbon, hard carbon, templated carbon,hollow carbon nanowires, hollow carbon sphere, natural graphite,artificial graphite, lithium or sodium titanate, NaTi₂(PO₄)₃, Na₂Ti₃O₇(Sodium titanate), Na₂C₈H₄O₄ (Disodium Terephthalate), Na₂TP (SodiumTerephthalate), TiO₂, Na_(x)TiO₂ (x=0.2 to 1.0), carboxylate basedmaterials, C₈H₄Na₂O₄, C₈H₆O₄, C₈H₅NaO₄, C₈Na₂F₄O₄, C₁₀H₂Na₄O₈, C₁₄H₄O₆,C₁₄H₄Na₄O₈, or a combination thereof. In an embodiment, the anode maycontain a mixture of 2 or 3 types of anode active materials (e.g. mixedparticles of activated carbon+NaTi₂(PO₄)₃ or a mixture of Li particlesand graphite particles).

The first or second liquid electrolyte in the invented process orbattery may be selected from an aqueous electrolyte, organicelectrolyte, ionic liquid electrolyte, mixture of an organic electrolyteand an ionic electrolyte, or a mixture thereof with a polymer. In someembodiments, the aqueous electrolyte contains a sodium salt or apotassium salt dissolved in water or a mixture of water and alcohol. Insome embodiments, the sodium salt or potassium salt is selected fromNa₂SO₄, K₂SO₄, a mixture thereof, NaOH, LiOH, NaCl, LiCl, NaF, LiF,NaBr, LiBr, NaI, LiI, or a mixture thereof.

A wide range of electrolytes can be used for practicing the instantinvention. Most preferred are non-aqueous organic and/or ionic liquidelectrolytes. The non-aqueous electrolyte to be employed herein may beproduced by dissolving an electrolytic salt in a non-aqueous solvent.Any known non-aqueous solvent which has been employed as a solvent for alithium secondary battery can be employed. A non-aqueous solvent mainlyconsisting of a mixed solvent comprising ethylene carbonate (EC) and atleast one kind of non-aqueous solvent whose melting point is lower thanthat of aforementioned ethylene carbonate and whose donor number is 18or less (hereinafter referred to as a second solvent) may be preferablyemployed. This non-aqueous solvent is advantageous in that it is (a)stable against a negative electrode containing a carbonaceous materialwell developed in graphite structure; (b) effective in suppressing thereductive or oxidative decomposition of electrolyte; and (c) high inconductivity. A non-aqueous electrolyte solely composed of ethylenecarbonate (EC) is advantageous in that it is relatively stable againstdecomposition through a reduction by a graphitized carbonaceousmaterial. However, the melting point of EC is relatively high, 39 to 40°C., and the viscosity thereof is relatively high, so that theconductivity thereof is low, thus making EC alone unsuited for use as asecondary battery electrolyte to be operated at room temperature orlower. The second solvent to be used in a mixture with EC functions tomake the viscosity of the solvent mixture lower than that of EC alone,thereby promoting the ion conductivity of the mixed solvent.Furthermore, when the second solvent having a donor number of 18 or less(the donor number of ethylene carbonate is 16.4) is employed, theaforementioned ethylene carbonate can be easily and selectively solvatedwith lithium ion, so that the reduction reaction of the second solventwith the carbonaceous material well developed in graphitization isassumed to be suppressed. Further, when the donor number of the secondsolvent is controlled to not more than 18, the oxidative decompositionpotential to the lithium electrode can be easily increased to 4 V ormore, so that it is possible to manufacture a lithium secondary batteryof high voltage.

Preferable second solvents are dimethyl carbonate (DMC), methylethylcarbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methylpropionate, propylene carbonate (PC), .gamma.-butyrolactone(.gamma.-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate(PF), methyl formate (MF), toluene, xylene and methyl acetate (MA).These second solvents may be employed singly or in a combination of twoor more. More desirably, this second solvent should be selected fromthose having a donor number of 16.5 or less. The viscosity of thissecond solvent should preferably be 28 cps or less at 25° C.

The mixing ratio of the aforementioned ethylene carbonate in the mixedsolvent should preferably be 10 to 80% by volume. If the mixing ratio ofthe ethylene carbonate falls outside this range, the conductivity of thesolvent may be lowered or the solvent tends to be more easilydecomposed, thereby deteriorating the charge/discharge efficiency. Morepreferable mixing ratio of the ethylene carbonate is 20 to 75% byvolume. When the mixing ratio of ethylene carbonate in a non-aqueoussolvent is increased to 20% by volume or more, the solvating effect ofethylene carbonate to lithium ions will be facilitated and the solventdecomposition-inhibiting effect thereof can be improved.

Examples of preferred mixed solvent are a composition comprising EC andMEC; comprising EC, PC and MEC; comprising EC, MEC and DEC; comprisingEC, MEC and DMC; and comprising EC, MEC, PC and DEC; with the volumeratio of MEC being controlled within the range of 30 to 80%. Byselecting the volume ratio of MEC from the range of 30 to 80%, morepreferably 40 to 70%, the conductivity of the solvent can be improved.With the purpose of suppressing the decomposition reaction of thesolvent, an electrolyte having carbon dioxide dissolved therein may beemployed, thereby effectively improving both the capacity and cycle lifeof the battery. The electrolytic salts to be incorporated into anon-aqueous electrolyte may be selected from a lithium salt such aslithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆),lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆),lithium trifluoro-metasulfonate (LiCF₃SO₃) and bis-trifluoromethylsulfonylimide lithium [LiN(CF₃SO₂)₂]. Among them, LiPF₆, LiBF₄ andLiN(CF₃SO₂)₂ are preferred.

The content of aforementioned electrolytic salts in the non-aqueoussolvent is preferably greater than 2.5 M (mol/l), more preferably >3.0M, further more preferably >5.0 M, still more preferably >7.0 M, andmost preferably >10 M. An electrolyte containing a higher concentrationof alkali metal salt makes it easier to form a rope-shape battery thatdoes not have the tendency to leak during manufacturing or duringbending or twisting of the cable battery. Further surprisingly, we haveobserved that most of the electrolytes become non-flammable when thesalt concentration exceeds 3.5M. Some becomes non-flammable at a saltconcentration greater than 3.0 M or just >2.5 M. Battery scientists andengineers would expect that higher concentration means higher viscosityand lower ion mobility and, hence, lower alkali ion conductivity. Wehave found that this trend is generally true of the salt concentrationrange of 0.01 M to 2.0 M. However, quite unexpectedly, the alkali ionconductivity (Li⁺, Na⁺, and K⁺ ions) begins to increase after theconcentration is increased to exceed a threshold level (typicallybetween 2.1 and 3.0 M). In other words, higher salt concentrations leadto both a higher ion conductivity (a surprise) and non-flammability(another surprise).

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, a salt is considered as an ionicliquid if its melting point is below 100° C. If the melting temperatureis equal to or lower than room temperature (25° C.), the salt isreferred to as a room temperature ionic liquid (RTIL). The IL salts arecharacterized by weak interactions, due to the combination of a largecation and a charge-delocalized anion. This results in a low tendency tocrystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combinationgives a fluid with an ionic conductivity comparable to many organicelectrolyte solutions and a low decomposition propensity and low vaporpressure up to ˜300-400° C. This implies a generally low volatility andnon-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic ions that come in anessentially unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide, andhexafluorophosphate as anions. Based on their compositions, ionicliquids come in different classes that basically include aprotic, proticand zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, butnot limited to, tetraalkylammonium, di-, tri-, andtetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH2CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻,N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.Relatively speaking, the combination of imidazolium- or sulfonium-basedcations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs withgood working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte ingredient (a salt and/or asolvent) in a battery.

The specific capacity and specific energy of a Li—S cell or Na—S cellare dictated by the actual amount of sulfur that can be implemented inthe cathode active layer (relative to other non-active ingredients, suchas the binder resin and conductive filler) and the utilization rate ofthis sulfur amount (i.e. the utilization efficiency of the cathodeactive material or the actual proportion of S that actively participatesin storing and releasing lithium ions). A high-capacity and high-energyLi—S or Na—S cell requires a high amount of S in the cathode activelayer (i.e. relative to the amounts of non-active materials, such as thebinder resin, conductive additive, and other modifying or supportingmaterials) and a high S utilization efficiency). The present inventionprovides such a sulfur or sulfide-containing cathode active layer and amethod of producing such a cathode active layer (e.g. a pre-sulfurizedactive cathode layer).

It may be noted that sulfur or a sulfur compound (e.g. particles oflithium polysulfide, sodium polysulfide, carbon-polymer compound, sulfurcarried by activated carbon particles, sulfur or sulfide carried ongraphene surfaces, etc.) may be incorporated into the pores of a foamstructure as described earlier and illustrated in FIG. 1(D) and FIG.1(E). Additionally and preferably, a highly innovative method may beused to incorporate sulfur or sulfur compound, with or without theeventual electrolyte, into a foam structure. Such a pre-sulfurizationmethod enables us to achieve both a high sulfur content and thin sulfurcoating/particle sizes, two features that were previously regarded asmutually exclusive. As an example of sulfur pre-loading procedures, thismethod comprises the following four steps, (a)-(d):

-   -   a) Preparing a layer of porous graphene/graphite/carbon        structure having pores and massive surfaces with a specific        surface area greater than 100 m²/g (these surfaces must be        accessible to electrolyte). The porous graphene/carbon/graphite        structure have a specific surface area preferably >500 m²/g and        more preferably >700 m²/g, and most preferably >1,000 m²/g. Many        types of graphene/carbon/graphite foam structures may be used,        including carbon foam, graphite foam, carbon aerogel foam,        graphene foam, graphene aerogel foam, electron-spun carbon foam,        carbon/graphite fiber mat, carbon/graphite fiber cloth,        carbon/graphite carbon paper, carbon nano-fiber mat/paper/cloth,        carbon nanotube mat/paper/cloth, activated carbon particles        (bonded together to form a foam, for instance), and exfoliated        graphite foam, etc.    -   b) Preparing an electrolyte comprising a solvent (e.g.        non-aqueous solvent, such as organic solvent and or ionic        liquid) and a sulfur source (e.g. metal polysulfide) dissolved        or dispersed in the solvent. This electrolyte can be the same        electrolyte as in the intended battery;    -   c) Preparing an anode;    -   d) Bringing the integral layer of porous        graphene/carbon/graphite structure and the anode in ionic        contact with the electrolyte (e.g. by immersing all these        components in a chamber that is external to the intended Li—S        cell, or encasing these three components inside the Li—S cell)        and imposing an electric current between the anode and the        integral layer of porous graphene/carbon/graphite structure        (serving as a cathode) with a sufficient current density for a        sufficient period of time to electrochemically deposit        nano-scaled sulfur particles or coating on the graphene surfaces        or internal pore walls of a graphite/carbon structure to form a        pre-sulfurized foam structure (e.g. a rod or layer);

The S particles or coating in the presently invented pre-sulfurized foamstructure typically have a thickness or diameter smaller than 20 nm(preferably and typically <10 nm, more preferably <5 nm, and furtherpreferably <3 nm) and wherein the nano-scaled sulfur particles orcoating occupy a weight fraction of at least 70% (preferably >80%, morepreferably >90%, and most preferably >95%) based on the total weights ofthe sulfur particles or coating and the graphene material combined. Itis advantageous to deposit as much S as possible yet still maintainultra-thin thickness or diameter of the S coating or particles(e.g. >80% and <3 nm; >90% and <5 nm; and >95% and <10 nm, etc.).

The rod of porous graphene/graphite/carbon structure recited in step (a)may contain a graphene material or an exfoliated graphite material,wherein the graphene material is selected from pristine graphene,graphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, boron-doped graphene, nitrogen-doped graphene,chemically functionalized graphene, or a combination thereof, andwherein the exfoliated graphite material is selected from exfoliatedgraphite worms, expanded graphite flakes, or recompressed graphite wormsor flakes (must still exhibit a high specific surface area, >>100 m²/g,accessible to electrolyte).

Once a layer of porous graphene/carbon/graphite structure is prepared,this layer can be immersed in an electrolyte (preferably liquidelectrolyte), which comprises a solvent and a sulfur source dissolved ordispersed in the solvent. This layer basically serves as a cathode in anexternal electrochemical deposition chamber.

Subsequently, an anode layer is also immersed in the chamber. Anyconductive material can be used as an anode material, but preferablythis layer contains some lithium or sodium. In such an arrangement, thelayer of porous graphene/carbon/graphite structure and the anode are inionic contact with the electrolyte. An electric current is then suppliedbetween the anode and the integral layer of porous graphene structure(serving as a cathode) with a sufficient current density for asufficient period of time to electrochemically deposit nano-scaledsulfur particles or coating on the graphene surfaces to form thepre-sulfurized active cathode layer. The required current densitydepends upon the desired speed of deposition and uniformity of thedeposited material.

This current density can be readily adjusted to deposit S particles orcoating that have a thickness or diameter smaller than 20 nm (preferably<10 nm, more preferably <5 nm, and further preferably <3 nm). Theresulting nano-scaled sulfur particles or coating occupy a weightfraction of at least 70% (preferably >80%, more preferably >90%, andmost preferably >95%) based on the total weights of the sulfur particlesor coating and the graphene material combined.

In one preferred embodiment, the sulfur source is selected fromM_(x)S_(y), wherein x is an integer from 1 to 3 and y is an integer from1 to 10, and M is a metal element selected from an alkali metal, analkaline metal selected from Mg or Ca, a transition metal, a metal fromgroups 13 to 17 of the periodic table, or a combination thereof. In adesired embodiment, the metal element M is selected from Li, Na, K, Mg,Zn, Cu, Ti, Ni, Co, Fe, or Al. In a particularly desired embodiment,M_(x)S_(y) is selected from Li₂S₆, Li₂S₇, Li₂S₈, Li₂S₉, Li₂S₁₀, Na₂S₆,Na₂S₇, Na₂S₈, Na₂S₉, Na₂S₁₀, K₂S₆, K₂S₇, K₂S₈, K₂S₉, or K₂S₁₀.

In one embodiment, the anode comprises an anode active material selectedfrom an alkali metal, an alkaline metal, a transition metal, a metalfrom groups 13 to 17 of the periodic table, or a combination thereof.This anode can be the same anode intended for inclusion in a Li—S cell.

The solvent and lithium or sodium salt used in the electrochemicaldeposition chamber may be selected from any solvent or salt in the listgiven above for a lithium-sulfur or sodium-sulfur battery.

After an extensive and in-depth research effort, we have come to realizethat such a pre-sulfurization surprisingly solves several most criticalissues associated with current Li—S or Na—S cells. For instance, thismethod enables the sulfur to be deposited in a thin coating orultra-fine particle form, thus, providing ultra-short lithium iondiffusion paths and, hence, ultra-fast reaction times for fast batterycharges and discharges. This is achieved while maintaining a relativelyhigh proportion of sulfur (the active material responsible for storinglithium) and, thus, high specific lithium storage capacity of theresulting cathode active layer in terms of high specific capacity(mAh/g, based on the total weight of the cathode layer, including themasses of the active material, S, supporting graphene sheets, binderresin, and conductive filler).

It is of significance to note that one might be able to use a prior artprocedure to deposit small S particles, but not a high S proportion, orto achieve a high proportion but only in large particles or thick filmform. But, the prior art procedures have not been able to achieve bothat the same time. This is why it is such an unexpected and highlyadvantageous thing to obtain a high sulfur loading and yet,concurrently, maintaining an ultra-thin/small thickness/diameter ofsulfur. This has not been possible with any prior art sulfur loadingtechniques. For instance, we have been able to deposit nano-scaledsulfur particles or coating that occupy a >90% weight fraction of thecathode layer and yet maintaining a coating thickness or particlediameter <3 nm. This is quite a feat in the art of lithium-sulfurbatteries. As another example, we have achieved a >95% S loading at anaverage S coating thickness of 4.8-7 nm.

Electrochemists or materials scientists in the art of Li—S batterieswould expect that a greater amount of highly conducting graphene sheetsor graphite flakes (hence, a smaller amount of S) in the cathode activelayer should lead to a better utilization of S, particularly under highcharge/discharge rate conditions. Contrary to these expectations, wehave observed that the key to achieving a high S utilization efficiencyis minimizing the S coating or particle size and is independent of theamount of S loaded into the cathode provided the S coating or particlethickness/diameter is small enough (e.g. <10 nm, or even better if <5nm). The problem here is that it has not been possible to maintain athin S coating or small particle size if S is higher than 50% by weight.Here we have further surprisingly observed that the key to enabling ahigh specific capacity at the cathode under high rate conditions is tomaintain a high S loading and still keep the S coating or particle sizeas small as possible, and this is accomplished by using the presentlyinvented pre-sulfurization method.

The electrons coming from or going out through the external load orcircuit must go through the conductive additives (in a conventionalsulfur cathode) or a conductive framework (e.g. exfoliated graphitemeso-porous structure or nano-structure of conductive graphene sheets asherein disclosed) to reach the cathode active material. Since thecathode active material (e.g. sulfur or lithium polysulfide) is a poorelectronic conductor, the active material particle or coating must be asthin as possible to reduce the required electron travel distance.

Furthermore, the cathode in a conventional Li—S cell typically has lessthan 70% by weight of sulfur in a composite cathode composed of sulfurand the conductive additive/support. Even when the sulfur content in theprior art composite cathode reaches or exceeds 70% by weight, thespecific capacity of the composite cathode is typically significantlylower than what is expected based on theoretical predictions. Forinstance, the theoretical specific capacity of sulfur is 1,675 mAh/g. Acomposite cathode composed of 70% sulfur (S) and 30% carbon black (CB),without any binder, should be capable of storing up to 1,675×70%=1,172mAh/g. Unfortunately, the observed specific capacity is typically lessthan 75% or 879 mAh/g (often less than 50% or 586 mAh/g in this example)of what could be achieved. In other words, the active materialutilization rate is typically less than 75% (or even <50%). This hasbeen a major issue in the art of Li—S cells and there has been nosolution to this problem. Most surprisingly, the implementation ofmassive graphene surfaces associated with a porous graphene structure asa conductive supporting material for sulfur or lithium polysulfide hasmade it possible to achieve an active material utilization rate oftypically >>80%, more often greater than 90%, and, in many cases, closeto 95%-99%.

Alternatively, the cathode active material (e.g. S or a sulfur compound)may be deposited on or bonded by a functional material ornano-structured material. The sulfur compound may be selected fromorgano-sulfur, polymer-sulfur, carbon-sulfur, metal sulfide, S—Sb, S—Bi,S—Se, S—Te mixture, or a combination thereof. These species may besupported by a conductive carrier particle, such as activated carbon orsmall graphene piece. The functional material or nano-structuredmaterial may be selected from the group consisting of (a) anano-structured or porous disordered carbon material selected from asoft carbon, hard carbon, polymeric carbon or carbonized resin,meso-phase carbon, coke, carbonized pitch, carbon black, activatedcarbon, nano-cellular carbon foam or partially graphitized carbon; (b) anano graphene platelet selected from a single-layer graphene sheet ormulti-layer graphene platelet; (c) a carbon nanotube selected from asingle-walled carbon nanotube or multi-walled carbon nanotube; (d) acarbon nano-fiber, nano-wire, metal oxide nano-wire or fiber, conductivepolymer nano-fiber, or a combination thereof; (e) a carbonyl-containingorganic or polymeric molecule; (f) a functional material containing acarbonyl, carboxylic, or amine group; and combinations thereof. In apreferred embodiment, the functional material or nano-structuredmaterial has a specific surface area of at least 500 m²/g, preferably atleast 1,000 m²/g.

Typically, the cathode active materials (sulfur or metal sulfide) arenot electrically conducting. Hence, in one embodiment, the cathodeactive material may be mixed with a conductive filler, such as carbonblack (CB), acetylene black (AB), graphite particles, expanded graphiteparticles, activated carbon, meso-porous carbon, meso-carbon micro bead(MCMB), carbon nano-tube (CNT), carbon nano-fiber (CNF), graphene sheet(also referred to as nano graphene platelet, NGP), carbon fiber, or acombination thereof. These carbon/graphite/graphene materials,containing sulfur or polysulfide, may be made into fine particles as thecathode active material to be incorporated in pores of the foamstructure in the invented Li—S or Na—S cell.

In a preferred embodiment, the nano-scaled filaments (e.g. CNTs, CNFs,and/or NGPs) are formed into a porous nano-structure that containsmassive surfaces to support either the anode active material (e.g. Na orLi coating) or the cathode active material (e.g. S). The porousnano-structure should have pores having a pore size preferably from 2 nmto 50 nm, preferably 2 nm-10 nm. These pores are properly sized toaccommodate the electrolyte at the cathode side and to retain thecathode active material in the pores during repeated charges/discharges.The same type of nano-structure may be implemented at the anode side tosupport the anode active material.

At the anode side, when an alkali metal is used as the sole anode activematerial in an alkali metal cell, there is concern about the formationof dendrites, which could lead to internal shorting and thermal runaway.Herein, we have used two approaches, separately or in combination, toaddressing this dendrite formation issue: one involving the use of ahigh-concentration electrolyte and the other the use of a nano-structurecomposed of conductive nano-filaments to support the alkali metal at theanode. The nano-filament may be selected from, as examples, a carbonnano fiber (CNF), graphite nano fiber (GNF), carbon nano-tube (CNT),metal nano wire (MNW), conductive nano-fibers obtained byelectro-spinning, conductive electro-spun composite nano-fibers,nano-scaled graphene platelet (NGP), or a combination thereof. Thenano-filaments may be bonded by a binder material selected from apolymer, coal tar pitch, petroleum pitch, meso-phase pitch, coke, or aderivative thereof.

Surprisingly and significantly, the nano-structure provides anenvironment that is conducive to uniform deposition of alkali metal ionsduring the battery re-charge, to the extent that no geometrically sharpstructures or dendrites were found in the anode after a large number ofcycles. Not wishing to be bound by any theory, but the applicantsenvision that the 3-D network of highly conductive nano-filamentsprovide a substantially uniform attraction of alkali metal ions backonto the filament surfaces during re-charging. Furthermore, due to thenanometer sizes of the filaments, there is a large amount of surfacearea per unit volume or per unit weight of the nano-filaments. Thisultra-high specific surface area offers the alkali metal ions anopportunity to uniformly deposit a thin coating on filament surfaces.The high surface area readily accepts a large amount of alkali metalions in the liquid electrolyte, enabling high re-charge rates for analkali metal secondary battery.

The presently invented rope-shaped battery has many unique features andsome of these features and advantages are summarized below:

By definition, a rope shape battery means a battery that contains atleast a rod-shape or filament-shape anode and a rod-shape orfilament-shape cathode combined into a braid or twist yarn. The batteryhas a length and a diameter or thickness wherein the aspect ratio(length-to-diameter or length-to-thickness ratio) is at least 10 andpreferably at least 20. The rope-shaped alkali metal battery can have alength greater than 1 m, or even greater than 100 m. The length can beas short as 1 μm, but typically from 10 μm to 10 m and more typicallyfrom micrometers to a few meters. Actually, there is no theoreticallimitation on the length of this type of rope-shape battery.

The invented rope-shaped alkali metal battery is so flexible that thebattery can be easily bent to have a radius of curvature greater than 10cm. The battery is bendable to substantially conform to the shape of avoid or interior compartment in a vehicle. The void or interiorcompartment may be a trunk, door, hatch, spare tire compartment, areaunder seat or area under dashboard. The battery is removable from avehicle and is bendable to conform to the shape of a different void orinterior compartment.

One or more units of instant rope-shape battery can be incorporated intoa garment, belt, luggage strap, weaponry strap, musical instrumentstrap, helmet, hat, boot, foot covering, glove, wrist covering, watchband, jewelry item, animal collar or animal harness.

One or more units of instant rope-shaped battery can be removablyincorporated a garment, belt, luggage strap, weaponry strap, musicalinstrument strap, helmet, hat, boot, foot covering, glove, wristcovering, watch band, jewelry item, animal collar or animal harness.

Additionally, the invented rope battery conforms to the interior radiusof a hollow bicycle frame.

In what follows, we provide examples for a large number of differenttypes of anode active materials, cathode active materials (from sulfurto various sulfur compounds), and conductive porous layers (e.g.graphite foam, graphene foam, and metal foam) to illustrate the bestmode of practicing the instant invention. Theses illustrative examplesand other portions of instant specification and drawings, separately orin combinations, are more than adequate to enable a person of ordinaryskill in the art to practice the instant invention. However, theseexamples should not be construed as limiting the scope of instantinvention.

Example 1: Illustrative Examples of Electronically Conductive PorousRods or Layers as an Active Material/Electrolyte-Accommodating CurrentCollector

Various types of metal foams, carbon foams, and fine metal webs/screensare commercially available for use as conductive porous rods or layersin an anode or cathode (serving as a current collector); e.g. Ni foam,Cu foam, Al foam, Ti foam, Ni mesh/web, stainless steel fiber mesh, etc.Metal-coated polymer foams and carbon foams are also used as currentcollectors. For making macroscopic cable-shaped flexible andshape-conformable batteries, the most desirable thickness ranges forthese conductive porous layers are 50-1000 μm, preferably 100-800 μm,more preferably 200-600 μm. For making microscopic cable-shape batteries(having a diameter from 100 nm to 100 μm, for instance), graphene foams,graphene aerogel foam, porous carbon fibers (e.g. made byelectro-spinning polymer fibers, carbonizing the polymer fibers, andactivating the resulting carbon fibers), and porous graphite fibers canbe used to accommodate the mixture of an electrode active material andelectrolyte.

Example 2: Ni Foam and CVD Graphene Foam-Based Porous Layers Supportedon Ni Foam Templates

The procedure for producing CVD graphene foam was adapted from thatdisclosed in open literature: Chen, Z. et al. “Three-dimensionalflexible and conductive interconnected graphene networks grown bychemical vapor deposition,” Nature Materials, 10, 424-428 (2011). Nickelfoam, a porous structure with an interconnected 3D scaffold of nickelwas chosen as a template for the growth of graphene foam. Briefly,carbon was introduced into a nickel foam by decomposing CH₄ at 1,000° C.under ambient pressure, and graphene films were then deposited on thesurface of the nickel foam. Due to the difference in the thermalexpansion coefficients between nickel and graphene, ripples and wrinkleswere formed on the graphene films. Four types of foams made in thisexample were used as a current collector in the presently inventedlithium batteries: Ni foam, CVD graphene-coated Ni form, CVD graphenefoam (Ni being etched away), and conductive polymer bonded CVD graphenefoam.

In order to recover (separate) graphene foam from the supporting Nifoam, Ni frame was etched away. In the procedure proposed by Chen, etal., before etching away the nickel skeleton by a hot HCl (or FeCl₃)solution, a thin layer of poly (methyl methacrylate) (PMMA) wasdeposited on the surface of the graphene films as a support to preventthe graphene network from collapsing during nickel etching. After thePMMA layer was carefully removed by hot acetone, a fragile graphene foamsample was obtained. The use of the PMMA support layer was consideredcritical to preparing a free-standing film of graphene foam. Instead, aconducting polymer was used as a binder resin to hold graphene togetherwhile Ni was etched away. The graphene foam or Ni foam thickness rangewas from 35 μm to 600 μm.

The layers of Ni foam or the CVD graphene foam used herein is intendedas conductive porous layers (CPL) to accommodate the ingredients (anodeor cathode active material+optional conductive additive+liquidelectrolyte) for the anode or cathode or both. For instance, Si nanoparticles or surface-stabilized Li particles dispersed in an organicliquid electrolyte (e.g. 1.0-5.5 M of LiPF₆ dissolved in PC-EC) weremade into gel-like mass, which was delivered to a porous surface of a Nifoam continuously fed from a feeder roller to make an anode electroderoller (as in Schematic A of FIG. 1(C)).

Graphene-supported lithium polysulfide nano particles dispersed in thesame liquid electrolyte were made into cathode slurry, which was sprayedover two porous surfaces of a continuous Ni foam layer to form a cathodeelectrode. A porous foam rod containing Si nano particle-electrolytemixture impregnated into the foam pores (the first electrode) waswrapped around by a porous separator layer (porous PE-PP copolymer),which in turn was wrapped around by a lithium sulfide-based cathodelayer. The cylindrical structure is then encased in a thin polymersheath to obtain a cable-shape lithium-ion battery.

Example 3: Graphitic Foam-Based Conductive Porous Layers fromPitch-Based Carbon Foams

Pitch powder, granules, or pellets are placed in a aluminum mold withthe desired final shape of the foam. Mitsubishi ARA-24 meso-phase pitchwas utilized. The sample is evacuated to less than 1 torr and thenheated to a temperature approximately 300° C. At this point, the vacuumwas released to a nitrogen blanket and then a pressure of up to 1,000psi was applied. The temperature of the system was then raised to 800°C. This was performed at a rate of 2 degree C./min. The temperature washeld for at least 15 minutes to achieve a soak and then the furnacepower was turned off and cooled to room temperature at a rate ofapproximately 1.5 degree C./min with release of pressure at a rate ofapproximately 2 psi/min. Final foam temperatures were 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns in a graphite crucible to 2500° C. and 2800° C. (graphitized) inArgon. The graphite foam layers are available in a thickness range of75-500 μm.

Example 4: Preparation of Graphene Oxide (GO) and Reduced Graphene Oxide(RGO) Nano Sheets from Natural Graphite Powder

Natural graphite from Huadong Graphite Co. (Qingdao, China) was used asthe starting material. GO was obtained by following the well-knownmodified Hummers method, which involved two oxidation stages. In atypical procedure, the first oxidation was achieved in the followingconditions: 1100 mg of graphite was placed in a 1000 mL boiling flask.Then, 20 g of K₂S₂O₈, 20 g of P₂O₅, and 400 mL of a concentrated aqueoussolution of H₂SO₄ (96%) were added in the flask. The mixture was heatedunder reflux for 6 hours and then let without disturbing for 20 hours atroom temperature. Oxidized graphite was filtered and rinsed withabundant distilled water until neutral pH. A wet cake-like material wasrecovered at the end of this first oxidation.

For the second oxidation process, the previously collected wet cake wasplaced in a boiling flask that contains 69 mL of a concentrated aqueoussolution of H₂SO₄ (96%). The flask was kept in an ice bath as 9 g ofKMnO₄ was slowly added. Care was taken to avoid overheating. Theresulting mixture was stirred at 35° C. for 2 hours (the sample colorturning dark green), followed by the addition of 140 mL of water. After15 min, the reaction was halted by adding 420 mL of water and 15 mL ofan aqueous solution of 30 wt % H₂O₂. The color of the sample at thisstage turned bright yellow. To remove the metallic ions, the mixture wasfiltered and rinsed with a 1:10 HCl aqueous solution. The collectedmaterial was gently centrifuged at 2700 g and rinsed with deionizedwater. The final product was a wet cake that contained 1.4 wt % of GO,as estimated from dry extracts. Subsequently, liquid dispersions of GOplatelets were obtained by lightly sonicating wet-cake materials, whichwere diluted in deionized water.

Surfactant-stabilized RGO (RGO-BS) was obtained by diluting the wet-cakein an aqueous solution of surfactants instead of pure water. Acommercially available mixture of cholate sodium (50 wt. %) anddeoxycholate sodium (50 wt. %) salts provided by Sigma Aldrich was used.The surfactant weight fraction was 0.5 wt. %. This fraction was keptconstant for all samples. Sonication was performed using a BransonSonifier S-250A equipped with a 13 mm step disruptor horn and a 3 mmtapered micro-tip, operating at a 20 kHz frequency. For instance, 10 mLof aqueous solutions containing 0.1 wt. % of GO was sonicated for 10 minand subsequently centrifuged at 2700 g for 30 min to remove anynon-dissolved large particles, aggregates, and impurities. Chemicalreduction of as-obtained GO to yield RGO was conducted by following themethod, which involved placing 10 mL of a 0.1 wt. % GO aqueous solutionin a boiling flask of 50 mL. Then, 10 μL of a 35 wt. % aqueous solutionof N₂H₄ (hydrazine) and 70 mL of a 28 wt. % of an aqueous solution ofNH₄OH (ammonia) were added to the mixture, which was stabilized bysurfactants. The solution was heated to 90° C. and refluxed for 1 h. ThepH value measured after the reaction was approximately 9. The color ofthe sample turned dark black during the reduction reaction.

RGO was used as a conductive additive in either or both of the anode andcathode active material in certain lithium batteries presently invented.Pre-lithiated RGO (e.g. RGO+lithium particles or RGO pre-deposited withlithium coating) was also used as an anode active material that wasmixed with a liquid electrolyte to form wet anode active materialmixtures for use in selected lithium-ion cells. Selected cathode activematerials (Li₂S nano particles) and non-lithiated RGO sheets weredispersed in a liquid electrolyte to prepare wet cathode active materialmixture. The wet anode active mixture and cathode active mixtures wereseparately delivered to surfaces of graphite foams for forming an anodelayer and a cathode layer, respectively. Cable-shape batteries were thenfabricated, wherein the core structure (first electrode) was either ananode or a cathode and the second electrode was a corresponding counterelectrode (either a cathode or an anode).

For comparison purposes, slurry coating and drying procedures wereconducted to produce conventional electrodes. Electrodes and a separatordisposed between two dried electrodes were then assembled and encased inan Al-plastic laminated packaging envelop, followed by liquidelectrolyte injection to form a conventional lithium battery cell.

Example 5: Preparation of Pristine Graphene Sheets (0% Oxygen)

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free, non-halogenated and halogen-free, etc.) can lead to aconductive additive having a high electrical and thermal conductivity.Pre-lithiated pristine graphene and pre-sodiated pristine graphene werealso used as an anode active material for a lithium-ion battery and asodium-ion battery, respectively. Pristine graphene sheets were producedby using the direct ultrasonication or liquid-phase production process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.Pristine graphene is essentially free from any non-carbon elements.

Pristine graphene sheets, as a conductive additive, along with an anodeactive material (or cathode active material in the cathode) were thenincorporated in a battery using both the presently invented procedureand conventional procedure of slurry coating, drying and layerlaminating. Both lithium-ion batteries and lithium metal batteries(impregnation into anode only) were investigated. Sodium-ion cells werealso prepared and studied.

Example 6: Preparation of Pre-Sodiated Graphene Fluoride Sheets as anAnode Active Material of a Sodium-Sulfur Battery

Several processes have been used by us to produce graphene fluoride(GF), but only one process is herein described as an example. In atypical procedure, highly exfoliated graphite (HEG) was prepared fromintercalated compound C₂F.xClF₃. HEG was further fluorinated by vaporsof chlorine trifluoride to yield fluorinated highly exfoliated graphite(FHEG). Pre-cooled Teflon reactor was filled with 20-30 mL of liquidpre-cooled ClF₃, the reactor was closed and cooled to liquid nitrogentemperature. Then, no more than 1 g of HEG was put in a container withholes for ClF₃ gas to access and situated inside the reactor. In 7-10days a gray-beige product with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol and ethanol, separately)and subjected to an ultrasound treatment (280 W) for 30 min, leading tothe formation of homogeneous yellowish dispersions. Upon removal ofsolvent, the dispersion became a brownish powder. The graphene fluoridepowder was mixed with sodium chips in a liquid electrolyte, allowing forpre-sodiation to occur before or after impregnation into pores of ananode current collector.

Example 7: Preparation of Nitrogenataed Graphene Nano Sheets and PorousGraphene Structures

Graphene oxide (GO), synthesized in Example 1, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s.

The product was washed several times with deionized water and vacuumdried. In this method graphene oxide gets simultaneously reduced anddoped with nitrogen. The products obtained with graphene:urea massratios of 1:0.5, 1:1 and 1:2 are designated as NGO-1, NGO-2 and NGO-3respectively and the nitrogen contents of these samples were 14.7, 18.2and 17.5 wt % respectively as found by elemental analysis. Thesenitrogenataed graphene sheets remain dispersible in water. Two types ofdispersions were then prepared. One involved adding water-solublepolymer (e.g. polyethylene oxide) into the nitrogenated graphenesheet-water dispersion to produce a water-based suspension. The otherinvolved drying the nitrogenated graphene sheet-water dispersion torecover nitrogenated graphene sheets, which were then added intoprecursor polymer-solvent solutions to obtain organic solvent-basedsuspensions.

The resulting suspensions were then cast, dried, carbonized andgraphitized to produce porous graphene structures. The carbonizationtemperatures for comparative samples are 900-1,350° C. Thegraphitization temperatures are from 2,200° C. to 2,950° C. The porousgraphene layers are used as the porous current collectors for both theanode and the cathode of Li—S cells.

Example 8: Conductive Web of Filaments from Electro-Spun PAA Fibrils asa Supporting Layer for the Anode

Poly (amic acid) (PAA) precursors for spinning were prepared bycopolymerizing of pyromellitic dianhydride (Aldrich) and4,4′-oxydianiline (Aldrich) in a mixed solvent oftetrahydrofurane/methanol (THF/MeOH, 8/2 by weight). The PAA solutionwas spun into fiber web using an electrostatic spinning apparatus. Theapparatus consisted of a 15 kV d.c. power supply equipped with thepositively charged capillary from which the polymer solution wasextruded, and a negatively charged drum for collecting the fibers.Solvent removal and imidization from PAA were performed concurrently bystepwise heat treatments under air flow at 40° C. for 12 h, 100° C. for1 h, 250° C. for 2 h, and 350° C. for 1 h. The thermally cured polyimide(PI) web samples were carbonized at 1,000° C. to obtain carbonizednano-fibers with an average fibril diameter of 67 nm. Such a web can beused as a conductive substrate for an anode active material. We observethat the implementation of a network of conductive nano-filaments at theanode of a Li—S or room temperature Na—S cell can effectively suppressthe initiation and growth of lithium or sodium dendrites that otherwisecould lead to internal shorting.

Example 9: Electrochemical Deposition of S on Various Webs or PorousStructures (External Electrochemical Deposition) for Li—S and Na—SBatteries

The electrochemical deposition may be conducted before the cathodeactive layer is incorporated into an alkali metal-sulfur battery cell(Li—S or Na—S cell). In this approach, the anode, the electrolyte, andthe integral layer of porous graphene structure (serving as a cathodelayer) are positioned in an external container outside of alithium-sulfur cell. The needed apparatus is similar to anelectro-plating system, which is well-known in the art.

In a typical procedure, a metal polysulfide (M_(x)S_(y)) is dissolved ina solvent (e.g. mixture of DOL/DME in a volume ratio from 1:3 to 3:1) toform an electrolyte solution. An amount of a lithium salt may beoptionally added, but this is not required for external electrochemicaldeposition. A wide variety of solvents can be utilized for this purposeand there is no theoretical limit to what type of solvents can be used;any solvent can be used provided that there is some solubility of themetal polysulfide in this desired solvent. A greater solubility wouldmean a larger amount of sulfur can be derived from the electrolytesolution.

The electrolyte solution is then poured into a chamber or reactor undera dry and controlled atmosphere condition (e.g. He or Nitrogen gas). Ametal foil can be used as the anode and a layer of the porous graphenefoam or carbon foam structure as the cathode; both being immersed in theelectrolyte solution. This configuration constitutes an electrochemicaldeposition system. The step of electrochemically depositing nano-scaledsulfur particles or coating on the graphene surfaces is conducted at acurrent density preferably in the range of 1 mA/g to 10 A/g, based onthe layer weight of the porous graphene structure.

The chemical reactions that occur in this reactor may be represented bythe following equation: M_(x)S_(y)→M_(x)S_(y-z)+zS (typically z=1-4).Quite surprisingly, the precipitated S is preferentially nucleated andgrown on massive graphene surfaces to form nano-scaled coating or nanoparticles. The coating thickness or particle diameter and the amount ofS coating/particles may be controlled by the specific surface area,electro-chemical reaction current density, temperature and time. Ingeneral, a lower current density and lower reaction temperature lead toa more uniform distribution of S and the reactions are easier tocontrol. A longer reaction time leads to a larger amount of S depositedon surfaces of pore walls and the reaction is ceased when the sulfursource is consumed or when a desired amount of S is deposited. TheseS-coated foam structures were then used as the cathode layer of acable-shape Li—S or Na—S cell.

Example 10: Chemical Reaction-Induced Deposition of Sulfur Particles inCarbon Nano-Fiber Mat

A chemical deposition method is herein utilized to prepare S coatingdeposited in the pores of a carbon nano-fiber mat. The procedure beganwith adding 0.58 g Na₂S into a flask that had been filled with 25 mldistilled water to form a Na₂S solution. Then, 0.72 g elemental S wassuspended in the Na₂S solution and stirred with a magnetic stirrer forabout 2 hours at room temperature. The color of the solution changedslowly to orange-yellow as the sulfur dissolved. After dissolution ofthe sulfur, a sodium polysulfide (Na₂S_(x)) solution was obtained(x=4-10).

Subsequently, a graphene-impregnated structure was prepared by achemical deposition method in an aqueous solution. A layer of nano-fibermat was dipped into the Na₂S, solution. Approximately 100 ml of 2 mol/LHCOOH solution was added into the Na₂S_(x) solution at a rate of 30-40drops/min and stirred for 2 hours. After deposition of S in the mat wasallowed to proceed for 3 hours, the porous mat structure was washed withacetone and distilled water several times to eliminate salts andimpurities. The reaction may be represented by the following reaction:S_(x) ²⁻+2H⁺→(x−1) S+H₂S. Subsequently, the S-impregnated mat was driedat 50° C. in a drying oven for 48 hours.

Example 11: Redox Chemical Reaction-Induced Deposition of Sulfur inGraphite Foam

In this chemical reaction-based deposition process, sodium thiosulfate(Na₂S₂O₃) was used as a sulfur source and HCl as a reactant. A rod ofgraphite foam was prepared and then immersed into a solution containingthe two reactants (HCl and Na₂S₂O₃). The reaction was allowed to proceedat 25-75° C. for 1-3 hours, leading to the precipitation of S particlesdeposited on surfaces of GO sheets. The reaction may be represented bythe following reaction:

2HCl+Na₂S₂O₃→2NaCl+S↓+SO₂↑+H₂O.

Example 12: Preparation of S-Impregnated Foam Via Solution Deposition

Sulfur powder was mixed and dispersed in a solvent (CS₂) to form asolution. A piece of Ni foam was immersed into the solution and,subsequently, the solvent was evaporated, allowing S to get precipitatedout to yield a S-impregnated structure.

Example 13: Graphene-Enhanced Nano Silicon Fabricated from TEOS as anAnode Active Material of a Lithium-Ion Battery

Dilute 1 wt. % N002-PS to 0.2 wt. % N002-PS by DI water, and place thediluted PS solution to the ultrasonic bath and ultrasonic process for 30minutes. Gradually add TEOS (0.2 wt. % N002-PS: TEOS=5:2) while stirringthe PS solution. Then, keep stirring for 24 hours to get a completehydrolysis of TEOS. Dropwise add 10% NH₃.H₂O till the formation of gel,and the gel can be called as TP gel. Grind the TP gel to tiny particles.Oven dries at 120° C. for 2 hours, at 150° C. for 4 hours. Mix the driedTP particles with Mg in a ratio of 10:7. Use 20 times amount of 7 mm SSballs and ball mill under Argon protection, gradually increase therotating speed to 250 rpm. Put certain amount of TPM powders in Nickelcrucible and heat treatment at 680° C. Prepare certain amount of 2M HClsolution. Then gradually add heat treated TPM powders to the acidsolution. Keep the reaction for 2-24 hours, and then put the turbidliquid to the ultrasonic bath and ultrasonic process for 1 hour. Pourout the suspension to the filtration system. Discard the bottom largeparticles. Use DI water to rinse three times. Dry the yellow paste andblend the yellow paste to powders. The as-prepared nano particle has aSSA value range of 30 m²/g to 200 m²/g due to different ratio ofgraphene contents

A certain amount of the dried TPM particles is then put into mufflefurnace and calcined at 400° C.˜600° C. for 2 hours under air purging toremove the carbon content from the nanocomposite, producinggraphene-free yellow-color silicon nano powders. Both Si nano powder andgraphene-wrapped Si nano particles were used as a high-capacity anodeactive material.

Example 14: Graphene-Enhanced Tin Oxide Particulates as an Anode ActiveMaterial

Tin oxide (SnO₂) nano particles, an anode active material, were obtainedby the controlled hydrolysis of SnCl₄.5H₂O with NaOH using the followingprocedure: SnCl₄.5H₂O (0.95 g, 2.7 m-mol) and NaOH (0.212 g, 5.3 m-mol)were dissolved in 50 mL of distilled water each. The NaOH solution wasadded drop-wise under vigorous stirring to the tin chloride solution ata rate of 1 mL/min. This solution was homogenized by sonication for 5min. Subsequently, the resulting hydrosol was reacted with the GOdispersion for 3 hours. To this mixed solution, few drops of 0.1 M ofH₂SO₄ were added to flocculate the product. The precipitated solid wascollected by centrifugation, washed with water and ethanol, and dried invacuum. The dried product was heat-treated at 400° C. for 2 h under Aratmosphere.

Example 15: Preparation and Electrochemical Testing of Various BatteryCells

For most of the anode and cathode active materials investigated, weprepared alkali metal-sulfur cells or alkali metal ion-sulfur cellsusing both the presently invented method and the conventional method.

With the conventional method, a typical anode composition includes 85wt. % active material (e.g., Sn- or Na₂C₈H₄O₄-coated graphene sheets forNa ion-sulfur anode; graphite or Si particles for Li ion-sulfur anode),7 wt. % acetylene black (Super-P), and 8 wt. % polyvinylidene fluoridebinder (PVDF, 5 wt. % solid content) dissolved inN-methyl-2-pyrrolidinoe (NMP). After coating the slurries on Cu foil,the electrodes were dried at 120° C. in vacuum for 2 h to remove thesolvent. Cathode layers are made in a similar manner (using Al foil asthe cathode current collector). An anode layer, separator layer (e.g.Celgard 2400 membrane), and a cathode layer are then laminated togetherand housed in a plastic-Al envelop. The cell is then injected with 1 MLiPF₆ or NaPF₆ electrolyte solution dissolved in a mixture of ethylenecarbonate (EC) and diethyl carbonate (DEC) (EC-DEC, 1:1 v/v). In somecells, ionic liquids were used as the liquid electrolyte. The cellassemblies were made in an argon-filled glove-box.

In the presently invented process, in certain examples, the anodecurrent collector (conductive porous structure for the anode), theseparator, and the cathode current collector (conductive porousstructure for the cathode side) are assembled in a protective housingbefore or after the injecting (or impregnation) of the first suspensionand/or the injecting (or impregnation) of the second suspension. In someexamples, we assembled an empty foamed anode current collector, a porousseparator layer, and an empty foamed current collector together to forman assembly that was housed in a pouch (made of Al-nylon bi-layer film).The first suspension was then injected into the anode current collectorand the second suspension was injected into the cathode currentcollector. The pouch was then sealed. In other examples, we impregnateda foamed anode current collector with the first suspension to form ananode layer and, separately, impregnated a foamed cathode currentcollector with the second suspension to form a cathode layer. The anodelayer, a porous separator layer, and the cathode layer were thenassembled and housed in a pouch to form a cell. With the instant method,typically no binder resin is needed or used, saving 8% weight (reducedamount of non-active materials).

The cyclic voltammetry (CV) measurements were carried out using an Arbinelectrochemical workstation at a typical scanning rate of 1 mV/s. Inaddition, the electrochemical performances of various cells were alsoevaluated by galvanostatic charge/discharge cycling at a current densityof from 50 mA/g to 10 A/g. For long-term cycling tests, multi-channelbattery testers manufactured by LAND were used.

It may be noted that, in lithium-ion battery industry, it is a commonpractice to define the cycle life of a battery as the number ofcharge-discharge cycles that the battery suffers 20% decay in capacitybased on the initial capacity measured after the requiredelectrochemical formation. The same definition for the cycle life of aLi—S or room temperature Na—S cell is herein followed.

Example 16: Representative Testing Results

For each sample, several current densities (representingcharge/discharge rates) were imposed to determine the electrochemicalresponses, allowing for calculations of energy density and power densityvalues required of the construction of a Ragone plot (power density vs.energy density). Shown in FIG. 5 are Ragone plots (gravimetric andvolumetric power density vs. energy density) of Na-ion battery cellscontaining hard carbon particles as the anode active material andactivated carbon/sulfur composite particles as the cathode activematerials. Two of the 4 data curves are for the cells prepared accordingto an embodiment of instant invention and the other two by theconventional slurry coating of electrodes (roll-coating of slurry).Several significant observations can be made from these data:

Both the gravimetric and volumetric energy densities and power densitiesof the room-temperature sodium ion-S battery cells prepared by thepresently invented method (denoted as “cable” in the figure legend) aresignificantly higher than those of their counterparts prepared via theconventional roll-coating method (denoted as “conventional”). A changefrom an anode thickness of 150 μm (coated on a flat solid Cu foil) to athickness of 225 μm (all accommodated in pores of a Ni foam having 85%porosity) and a corresponding change in the cathode to maintain abalanced capacity ratio results in a gravimetric energy density increasefrom 155 Wh/kg to 185 Wh/kg. Even more surprisingly, the volumetricenergy density is increased from 232 Wh/L to 314 Wh/L.

These significant differences cannot be simply ascribed to the increasesin the electrode thickness and the mass loading. The differences arelikely due to the significantly higher active material mass loading(relative to other materials) associated with the presently inventedcells, reduced proportion of overhead (non-active) components relativeto the active material weight/volume, no need to have a binder resin,surprisingly better utilization of the electrode active material (most,if not all, of the hard carbon particles and C/S particles contributingto the sodium ion storage capacity; no dry pockets or ineffective spotsin the electrode, particularly under high charge/discharge rateconditions), and the surprising ability of the inventive method to moreeffectively pack active material particles in the pores of the foamedcurrent collector.

FIG. 6 shows the Ragone plots (both gravimetric and volumetric powerdensity vs. gravimetric and volumetric energy density) of two cells,both containing graphene-embraced Na nano particles as the anode activematerial and S impregnated in pores of graphene foam as the cathodeactive material. The experimental data were obtained from the batterycells that were prepared by the presently invented method into a cableconfiguration and those by the conventional slurry coating ofelectrodes.

These data indicate that both the gravimetric and volumetric energydensities and power densities of the battery cells prepared by thepresently invented method are significantly higher than those of theircounterparts prepared via the conventional method. The differences arehuge. The conventionally made cells exhibit a gravimetric energy densityof 215 Wh/kg and volumetric energy density of 323 Wh/L, but thepresently invented cable-shaped cells deliver 338 Wh/kg and 575 Wh/L,respectively. The cell-level volumetric energy density of 575 Wh/L hasnever been previously achieved with any rechargeable sodium batteries.The power densities as high as 1434 W/kg and 2,580 W/L are alsounprecedented for typically higher-energy lithium-ion batteries, letalone for sodium-ion batteries.

These energy density and power density differences are mainly due to thehigh active material mass loading (>25 mg/cm² in the anode and >30mg/cm² in the cathode) associated with the presently invented cells,reduced proportion of overhead (non-active) components relative to theactive material weight/volume, no need to have a binder resin, theability of the inventive method to better utilize the active materialparticles (all particles being accessible to liquid electrolyte and fastion and electron kinetics), and to more effectively pack active materialparticles in the pores of the foamed current collectors.

Shown in FIG. 7 are Ragone plots of Li—S batteries containing a lithiumfoil as the anode active material, S impregnated in graphite foam as thecathode active material, and lithium salt (LiaPF₆)-PC/DEC as organicliquid electrolyte. The data are for both lithium metal cells preparedby the presently invented method (cable-shaped cells) and those by theconventional slurry coating of electrodes. These data indicate that boththe gravimetric and volumetric energy densities and power densities ofthe sodium metal cells prepared by the presently invented method aresignificantly higher than those of their counterparts prepared via theconventional method. Again, the differences are huge and are likely dueto the significantly higher active material mass loading associated withthe presently invented cells, reduced proportion of overhead(non-active) components relative to the active material weight/volume,no need to have a binder resin, surprisingly better utilization of theelectrode active material (most, if not all, of the active materialcontributing to the lithium ion storage capacity; no dry pockets orineffective spots in the electrode, particularly under highcharge/discharge rate conditions), and the surprising ability of theinventive method to more effectively pack active material particles inthe pores of the foamed current collector.

Quite noteworthy and unexpected is the observation that the cell-levelgravimetric energy density of the presently invented Li—S cell is ashigh as 643 Wh/kg, higher than those of all rechargeable lithium metalor lithium-ion batteries ever reported (recall that current Li-ionbatteries typically store 150-250 Wh/kg based on the total cell weightand 500-650 Wh/L per cell volume). Furthermore, for sulfur cathodeactive material-based lithium batteries, a volumetric energy density of1,185 Wh/L, a gravimetric power density of 2,450 W/kg and volumetricpower density of 4,423 W/L would have been un-thinkable.

It is of significance to point out that reporting the energy and powerdensities per weight of active material alone on a Ragone plot, as didby many researchers, may not give a realistic picture of the performanceof the assembled battery cell. The weights of other device componentsalso must be taken into account. These overhead components, includingcurrent collectors, electrolyte, separator, binder, connectors, andpackaging, are non-active materials and do not contribute to the chargestorage amounts. They only add weights and volumes to the device. Hence,it is desirable to reduce the relative proportion of overhead componentweights and increase the active material proportion. However, it has notbeen possible to achieve this objective using conventional batteryproduction processes. The present invention overcomes thislong-standing, most serious problem in the art of lithium batteries.

In commercial lithium-ion batteries having an electrode thickness of 150μm, the weight proportion of the anode active material (e.g. graphite orcarbon) in a lithium-ion battery is typically from 12% to 17%, and thatof the cathode active material (for inorganic material, such as LiMn₂O₄)from 22% to 41%, or from 10% to 15% for organic or polymeric. Thecorresponding weight fractions in Na-ion batteries are expected to bevery similar since both the anode active materials and cathode activematerials have similar physical densities between two types of batteriesand the ratio of cathode specific capacity to the anode specificcapacity is also similar. Hence, a factor of 3 to 4 may be used toextrapolate the energy or power densities of the device (cell) from theproperties based on the active material weight alone. In most of thescientific papers, the properties reported are typically based on theactive material weight alone and the electrodes are typically very thin(<<100 μm and mostly <<50 μm). The active material weight is typicallyfrom 5% to 10% of the total device weight, which implies that the actualcell (device) energy or power densities may be obtained by dividing thecorresponding active material weight-based values by a factor of 10 to20. After this factor is taken into account, the properties reported inthese papers do not really look any better than those of commercialbatteries. Thus, one must be very careful when it comes to read andinterpret the performance data of batteries reported in the scientificpapers and patent applications.

FIG. 8 shows the Ragone plot of a series of Li ion-S cells(graphene-wrapped Si nano particles, or pre-lithiated Si nano particles)prepared by the conventional slurry coating process and the Ragone plotof corresponding cells prepared by the presently invented process. Thesedata again demonstrate the effectiveness of the presently inventedprocess in imparting unexpectedly high energy densities, bothgravimetric and volumetric, to the Li—S battery cells.

Example 17: Achievable Electrode Diameter or Thickness and its Effect onElectrochemical Performance of Lithium-Sulfur Battery Cells

One might be tempted to think the electrode thickness of an alkalimetal-sulfur battery is a design parameter that can be freely adjustedfor optimization of device performance. Contrary to this perception, inreality, the electrode thickness of an alkali metal battery (includingalkali metal-sulfur) is manufacturing-limited and one cannot produceelectrodes of good structural integrity that exceed certain thicknesslevel in a real industrial manufacturing environment (e.g. aroll-to-roll slurry coating facility). The conventional batteryelectrode design is based on coating an electrode layer on a flat metalcurrent collector, which has several major problems: (a) A thick coatingon Cu foil or Al foil requires a long drying time (requiring a heatingzone 30-100 meters long). (b) Thick electrodes tend to get delaminatedor cracked upon drying and subsequent handling, and even with a resinbinder proportion as high as 15-20% to hopefully improve the electrodeintegrity this problem remains a major limiting factor. Thus, such anindustry practice of roll-coating of slurry on a solid flat currentcollector does not allow for high active material mass loadings. (c) Athick electrode prepared by coating, drying, and compression makes itdifficult for electrolyte (injected into a cell after the cell is made)to permeate through the electrode and, as such, a thick electrode wouldmean many dry pockets or spots that are not wetted by the electrolyte.This would imply a poor utilization of the active materials. The instantinvention solves these long-standing, critically important issuesassociated with alkali metal batteries.

Shown in FIG. 9 are the cell-level gravimetric (Wh/kg) and volumetricenergy densities (Wh/L) of Li ion-S cells (Pre-lithiated graphiteanode+RGO foam-supported S cathode) plotted over the achievable cathodethickness range of the S/RGO cathode prepared via the conventionalmethod without delamination and cracking and those by the presentlyinvented cable construction method.

The electrodes can be fabricated up to a thickness of 100-200 μm usingthe conventional slurry coating process. However, in contrast, there isno theoretical limit on the electrode thickness or diameter that can beachieved with the presently invented method. Typically, the practicalelectrode thickness is from 10 μm to 1000 μm, more typically from 100 μmto 800 μm, and most typically from 200 μm to 600 μm.

These data further confirm the surprising effectiveness of the presentlyinvented method in producing ultra-thick lithium or sodium batteryelectrodes not previously achievable. These ultra-thick electrodes insodium metal batteries lead to exceptionally high sulfur cathode activematerial mass loading, typically significantly >15 mg/cm² (moretypically >20 mg/cm², further typically >30 mg/cm², often >40 mg/cm²,and even >50 mg/cm²). These high active material mass loadings have notbeen possible to obtain with conventional alkali metal-sulfur batteriesmade by the slurry coating processes. These high active material massloadings led to exceptionally high gravimetric and volumetric energydensities that otherwise have not been previously achieved given thesame battery system.

Dendrite issues commonly associated with Li and Na metal secondary cellsare also resolved by using the presently invented foamed currentcollector strategy. Hundreds of cells have been investigated and thosecells having a foamed anode current collector were not found to fail dueto dendrite penetration through the separator. SEM examination ofsamples from presently invented sodium and potassium cells confirms thatthe re-deposited alkali metal surfaces on pore walls in a porous anodecurrent collector appear to be smooth and uniform, exhibiting no sign ofsharp metal deposit or tree-like features as often observed withcorresponding cells having a solid current collector (Cu foil) at theanode. This might be due to a reduced exchange current densityassociated with a high specific surface area of the foamed currentcollector at the anode and a more uniform local electric field in such afoamed structure that drives the alkali metal deposition during repeatedre-charge procedures.

We claim:
 1. A process for producing a rope-shaped alkali metal-sulfurbattery wherein said alkali metal is selected from Li, Na, or acombination thereof; said process comprising: (a) providing a firstelectrode comprising a first electrically conductive porous rod havingpores and a first mixture of a first electrode active material and afirst electrolyte, wherein said first mixture resides in said pores ofsaid first porous rod; (b) wrapping or encasing a porous separatoraround said first electrode to form a separator-protected firstelectrode; (c) providing a second electrode comprising a secondelectrically conductive porous rod having pores and a second mixture ofa second electrode active material and a second electrolyte, whereinsaid second mixture resides in said pores of said second porous rod; (d)combining or interlacing said separator-protected first electrode andsaid second electrode to form a braid or a yarn having a twist or spiralelectrode, wherein said first electrode and second electrode contain ananode and a cathode; and (e) wrapping or encasing a protective casing orsheath around said braid or yarn to form said rope-shaped battery;wherein either the first electrode or the second electrode is a cathodecontaining sulfur or a sulfur compound as a cathode active material andsaid battery has a rope shape having a length-to-diameter orlength-to-thickness aspect ratio no less than
 5. 2. The process of claim1, wherein said sulfur compound is selected from organo-sulfur,polymer-sulfur, carbon-sulfur, metal sulfide, S—Sb, S—Bi, S—Se, S—Temixture, and combinations thereof.
 3. The process of claim 1, whereinsaid cathode active material contains a material selected from the groupconsisting of sulfur bonded to pore walls of said first or second porousrod, sulfur bonded to or confined by a carbon or graphite material,sulfur bonded to or confined by a polymer, sulfur-carbon compound, metalsulfide M_(x)S_(y), wherein x is an integer from 1 to 3 and y is aninteger from 1 to 10, and M is a metal element selected from Li, Na, K,Mg, Ca, a transition metal, a metal from groups 13 to 17 of the periodictable, and combinations thereof.
 4. The process of claim 1, wherein saidfirst electrode is a negative electrode or anode and said secondelectrode is a positive electrode or cathode.
 5. The process of claim 1,wherein said second electrode is a negative electrode or anode and saidfirst electrode is a positive electrode or cathode.
 6. A process forproducing a rope-shaped alkali metal-sulfur battery wherein said alkalimetal is selected from Li, Na, or a combination thereof said processcomprising: (a) providing a first electrode comprising a firstelectrically conductive rod and a first mixture of a first electrodeactive material and a first electrolyte, wherein said first mixture isdeposited on or in said first rod; (b) wrapping or encasing a porousseparator around said first electrode to form a separator-protectedfirst electrode; (c) providing a second electrode comprising a secondelectrically conductive porous rod having pores and a second mixture ofa second electrode active material and a second electrolyte, whereinsaid second mixture resides in said pores of said second porous rod; (d)combining said separator-protected first electrode and said secondelectrode in an interlacing or twisting manner to form a braid or yarn;and (e) wrapping or encasing a protective casing or sheath around saidbraid or yarn; wherein either the first electrode or the secondelectrode is a cathode and either the first electrode active material orthe second electrode active material is a cathode active materialselected from sulfur bonded to pore walls of said porous rod, sulfurbonded to or confined by a carbon or graphite material, sulfur bonded toor confined by a polymer, sulfur-carbon compound, metal sulfideM_(x)S_(y), wherein x is an integer from 1 to 3 and y is an integer from1 to 10, and M is a metal element selected from Li, Na, K, Mg, Ca, atransition metal, a metal from groups 13 to 17 of the periodic table, ora combination thereof and said battery has a rope shape having alength-to-diameter or length-to-thickness aspect ratio no less than 5.7. A process for producing a rope-shaped alkali metal-sulfur batterywherein said alkali metal is selected from Li, Na, or a combinationthereof; said process comprising: a) providing a first electrodecomprising an electrically conductive porous rod having at least 50% byvolume of pores and a first mixture of a first electrode active materialand a first electrolyte wherein said first mixture resides in said poresof said porous rod; b) wrapping a porous separator around said firstelectrode to form a separator-protected first electrode; c) providing asecond electrode comprising an electrically conductive rod having asecond mixture of a second electrode active material and a secondelectrolyte deposited thereon or therein; d) combining saidseparator-protected first electrode and said second electrode to form abraid or twist yarn; and e) wrapping or encasing a protective casing orsheath around said braid or yarn to form said rope-shaped battery;wherein either the first electrode or the second electrode is a cathodecontaining a cathode active material selected from sulfur or a sulfurcompound selected from organo-sulfur, polymer-sulfur, carbon-sulfur,metal sulfide, S—Sb, S—Bi, S—Se, S—Te mixture, or a combination thereofand said battery has a rope shape having a length-to-diameter orlength-to-thickness aspect ratio no less than
 5. 8. The process of claim1, wherein said cathode active material is supported by a functionalmaterial or nano-structured material selected from the group consistingof: i. a nano-structured or porous disordered carbon material selectedfrom particles of a soft carbon, hard carbon, polymeric carbon orcarbonized resin, meso-phase carbon, coke, carbonized pitch, carbonblack, activated carbon, nano-cellular carbon foam or partiallygraphitized carbon; ii. a nano graphene platelet selected from asingle-layer graphene sheet or multi-layer graphene platelet; iii. acarbon nanotube selected from a single-walled carbon nanotube ormulti-walled carbon nanotube; iv. a carbon nano-fiber, nano-wire, metaloxide nano-wire or fiber, conductive polymer nano-fiber, or acombination thereof; v. a carbonyl-containing organic or polymericmolecule; vi. a functional material containing a carbonyl, carboxylic,or amine group to reversibly capture sulfur; and combinations thereof.9. The process of claim 6, further comprising a porous separatorwrapping around or encasing said second electrode to form aseparator-protected second electrode.
 10. The process of claim 1,further comprising a step of disposing a third electrolyte between saidbraid or yarn and said protective sheath.
 11. The process of claim 1,wherein step (a) further includes introducing at least one metallicwire, conductive carbon/graphite fiber, or conductive polymer fiber intosaid conductive porous rod.
 12. The process of claim 1, wherein saidfirst or second electrically conductive porous rod contains a porousfoam selected from the group consisting of metal foam, metal web, metalfiber mat, metal nanowire mat, conductive polymer fiber mat, conductivepolymer foam, conductive polymer-coated fiber foam, carbon foam,graphite foam, carbon aerogel, carbon xerogel, graphene aerogel,graphene foam, graphene oxide foam, reduced graphene oxide foam, carbonfiber foam, graphite fiber foam, exfoliated graphite foam, andcombinations thereof.
 13. The process of claim 12, wherein said porousfoam has a cross-section that is circular, elliptic, rectangular,square, hexagon, hollow, or irregular in shape.
 14. The process of claim1, wherein said battery has a rope shape having a length/thickness orlength/diameter ratio greater than
 10. 15. The process of claim 1,wherein said step (a) or step (c) includes a procedure of introducingparticles, foil, or coating of Li, Na, K, or a combination thereof as anelectrode active material into said first electrode or said secondelectrode.
 16. The process of claim 1, wherein said step (a) includes(i) an operation of continuously feeding said electrically conductiveporous rod to a first electrode active material impregnation zone,wherein said conductive porous rod contains interconnectedelectron-conducting pathways and has at least one porous surface; and(ii) an operation of impregnating said first mixture into saidelectrically conductive porous rod from said at least one porous surfaceto form said first electrode.
 17. The process of claim 16, wherein saidstep (a) includes delivering, continuously or intermittently on demand,said first mixture to said at least one porous surface through spraying,printing, coating, casting, conveyor film delivery, and/or rollersurface delivery.
 18. The process of claim 1, wherein said step (c)includes (i) an operation of continuously feeding said electricallyconductive porous rod to an impregnation zone for said second electrodeactive material, wherein said conductive porous rod containsinterconnected electron-conducting pathways and has at least one poroussurface; and (ii) an operation of impregnating said second mixture intosaid electrically conductive porous rod from said at least one poroussurface to form said second electrode.
 19. The process of claim 18,wherein said step (c) includes delivering, continuously orintermittently on demand, said second mixture to said at least oneporous surface through spraying, printing, coating, casting, conveyorfilm delivery, and/or roller surface delivery.
 20. The process of claim1, wherein said step (b) contains wrapping around said first electrodewith a porous separator band in a coiled or spiral manner to form saidporous separator-protected first electrode.
 21. The process of claim 1,wherein said step (b) contains spraying an electrically insulatingmaterial to encase said first electrode, forming a porous shellstructure covering said first electrode to form said porousseparator-protected structure.
 22. The process of claim 1, wherein saidalkali metal battery is a lithium-ion battery and said first or secondelectrode active material is selected from the group consisting of: (a)particles of natural graphite, artificial graphite, meso-carbonmicrobeads (MCMB), and carbon; (b) silicon (Si), germanium (Ge), tin(Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al),nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), andcadmium (Cd); (c) alloys or intermetallic compounds of Si, Ge, Sn, Pb,Sb, Bi, Zn, Al, or Cd with other elements, wherein said alloys orcompounds are stoichiometric or non-stoichiometric; (d) oxides,carbides, nitrides, sulfides, phosphides, selenides, and tellurides ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and theirmixtures or composites; (e) pre-lithiated versions thereof; (f)pre-lithiated graphene sheets; and combinations thereof.
 23. The processof claim 22, wherein said pre-lithiated graphene sheets are selectedfrom the group consisting of pre-lithiated versions of pristinegraphene, graphene oxide, reduced graphene oxide, graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, boron-doped graphene, nitrogen-dopedgraphene, chemically functionalized graphene, a physically or chemicallyactivated or etched version thereof, and combinations thereof.
 24. Theprocess of claim 1, wherein said alkali metal battery is a sodium-ionbattery and said first or second electrode active material contains analkali intercalation compound selected from the group consisting ofpetroleum coke, carbon black, amorphous carbon, activated carbon, hardcarbon, soft carbon, templated carbon, hollow carbon nanowires, hollowcarbon sphere, titanates, NaTi₂(PO₄)₃, Na₂Ti₃O₇, Na₂C₈H₄O₄, Na₂TP,Na_(x)TiO₂ (x=0.2 to 1.0), Na₂C₈H₄O₄, carboxylate based materials,C₈H₄Na₂O₄, C₈H₆O₄, C₈H₅NaO₄, C₈Na₂F₄O₄, C₁₀H₂Na₄O₈, C₁₄H₄O₆, C₁₄H₄Na₄O₈,and combinations thereof.
 25. The process of claim 1, wherein saidalkali metal battery is a sodium-ion battery and said first or secondelectrode active material contains an alkali intercalation compoundselected from the group consisting of: (a) sodium- or potassium-dopedsilicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb),bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), cobalt (Co),nickel (Ni), manganese (Mn), cadmium (Cd), and mixtures thereof; (b)sodium- or potassium-containing alloys or intermetallic compounds of Si,Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c)sodium- or potassium-containing oxides, carbides, nitrides, sulfides,phosphides, selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof;(d) sodium or potassium salts; (e) graphene sheets pre-loaded withsodium or potassium; and combinations thereof.
 26. The process of claim1, wherein said first electrolyte and/or said second electrolytecontains a lithium salt or sodium salt dissolved in a liquid solvent andwherein said liquid solvent is water, an organic solvent, an ionicliquid, a mixture of an organic solvent and an ionic liquid, or a liquidsolvent-polymer gel.
 27. The process of claim 1, wherein said firstelectrolyte and/or said second electrolyte contains a lithium salt orsodium salt dissolved in a liquid solvent having a salt concentrationfrom 2.5 M to 15 M.
 28. The process of claim 1, wherein said first orsecond electrically conductive porous rod has from 70% to 99% by volumeof pores.